Pyth, 14 bytes

Shorter than I expected, but I'm doing something wrong.

V_UG+*dNrG1=tG

Explanation coming when I figure it out.

Pyth, 16 bytes

V26p*-25Nd>_rG1N

Explanation:

V26        26 times, could've written lG, but it's the same
   p*-25Nd Print without newline: 25-index spaces (could've used + instead of p but it's simpler to explain)
   >_rG1N  Implicit print with newline: The last index characters of the reverse uppercase alphabet

Try it online!

While you're here, here's Artemis's first answer ported into Pyth:

Pyth, 21 20 bytes (non-competing)

J26W=tJ+*dJ>_rG1-25J

An explanation for Artemis's answer:

J26        Set J to 26
W          While...
 =tJ      Decrement J (J-=1)
 +*dJ      J spaces plus...
 >_rG1-25J The last 25-index characters of the reverse uppercase alphabet

I should stop writing these on my phone

Try it online!

Pyth, 14 bytes

-2 from dave and ven thanks guys

V_UG+*dNrG1=PG

Explanation coming when I figure it out wake up.

Pyth, 16 bytes

V26p*-25Nd>_rG1N

Explanation:

V26        26 times, could've written lG, but it's the same
   p*-25Nd Print without newline: 25-index spaces (could've used + instead of p but it's simpler to explain)
   >_rG1N  Implicit print with newline: The last index characters of the reverse uppercase alphabet

Try it online!

While you're here, here's Artemis's first answer ported into Pyth:

Pyth, 21 20 bytes (non-competing)

J26W=tJ+*dJ>_rG1-25J

An explanation for Artemis's answer:

J26        Set J to 26
W          While...
 =tJ       Decrement J (J-=1)
 +*dJ      J spaces plus...
 >_rG1-25J The last 25-index characters of the reverse uppercase alphabet

I should stop writing these on my phone

Try it online!

Pyth, 16 bytes

Shorter than I expected, but I'm doing something wrong.

V26p*-25Nd>_rG1N

Explanation:

V26        26 times, could've written lG, but it's the same
   p*-25Nd Print without newline: 25-index spaces (could've used + instead of p but it's simpler to explain)
   >_rG1N  Implicit print with newline: The last index characters of the reverse uppercase alphabet

Try it online!

While you're here, here's Artemis's first answer ported into Pyth:

Pyth, 21 bytes (non-competing)

J26W=JtJ+*dJ>_rG1-25J

An explanation for Artemis's answer:

J26        Set J to 26
W          While...
 =JtJ      Decrement J (J=J-1)
 +*dJ      J spaces plus...
 >_rG1-25J The last 25-index characters of the reverse uppercase alphabet

I should stop writing these on my phone

Try it online!

Pyth, 14 bytes

Shorter than I expected, but I'm doing something wrong.

V_UG+*dNrG1=tG

Explanation coming when I figure it out.

Pyth, 16 bytes

V26p*-25Nd>_rG1N

Explanation:

V26        26 times, could've written lG, but it's the same
   p*-25Nd Print without newline: 25-index spaces (could've used + instead of p but it's simpler to explain)
   >_rG1N  Implicit print with newline: The last index characters of the reverse uppercase alphabet

Try it online!

While you're here, here's Artemis's first answer ported into Pyth:

Pyth, 21 20 bytes (non-competing)

J26W=tJ+*dJ>_rG1-25J

An explanation for Artemis's answer:

J26        Set J to 26
W          While...
 =tJ      Decrement J (J-=1)
 +*dJ      J spaces plus...
 >_rG1-25J The last 25-index characters of the reverse uppercase alphabet

I should stop writing these on my phone

Try it online!

Pyth, 16 bytes

Shorter than I expected, but I'm doing something wrong.

V26p*-25Nd>_rG1N

Explanation (if anybody wants it):

V26        26 times, could've written lG, but it's the same
   p*-25Nd  Print without newline: 25-index spaces (could've used + instead of p but it's simpler to explain)
   >_rG1N Implicit print with newline: The last index characters of the reverse uppercase alphabet

Try it online!

While you're here, here's Artemis's first answer ported into Pyth:

Pyth, 21 bytes (non-competing, then again, which of mine do compete?)

J26W=JtJ+*dJ>_rG1-25J

Another explanation if you haven't scrolled off by now:

J26        Set J to 26
W          While...
 =JtJ      Decrement J (J=J-1)
 +*dJ      J spaces plus...
 >_rG1-25J The last 25-index characters of the reverse uppercase alphabet

I should stop writing these on my phone

Try it online!

Pyth, 16 bytes

Shorter than I expected, but I'm doing something wrong.

V26p*-25Nd>_rG1N

Explanation:

V26        26 times, could've written lG, but it's the same
   p*-25Nd Print without newline: 25-index spaces (could've used + instead of p but it's simpler to explain)
   >_rG1N  Implicit print with newline: The last index characters of the reverse uppercase alphabet

Try it online!

While you're here, here's Artemis's first answer ported into Pyth:

Pyth, 21 bytes (non-competing)

J26W=JtJ+*dJ>_rG1-25J

An explanation for Artemis's answer:

J26        Set J to 26
W          While...
 =JtJ      Decrement J (J=J-1)
 +*dJ      J spaces plus...
 >_rG1-25J The last 25-index characters of the reverse uppercase alphabet

I should stop writing these on my phone

Try it online!