05AB1E, 4 bytes
AƶJΩ
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AƶJΩ # full program
Ω # random character from...
J # joined...
A # "abcdefghijklmnopqrstuvwxyz"...
ƶ # with each character repeated 1-indexed index of character in string times
# implicit output
AƶJ generates the string 'abbcccddddeeeeeffffffggggggghhhhhhhhiiiiiiiiijjjjjjjjjjkkkkkkkkkkkllllllllllllmmmmmmmmmmmmmnnnnnnnnnnnnnnoooooooooooooooppppppppppppppppqqqqqqqqqqqqqqqqqrrrrrrrrrrrrrrrrrrsssssssssssssssssssttttttttttttttttttttuuuuuuuuuuuuuuuuuuuuuvvvvvvvvvvvvvvvvvvvvvvwwwwwwwwwwwwwwwwwwwwwwwxxxxxxxxxxxxxxxxxxxxxxxxyyyyyyyyyyyyyyyyyyyyyyyyyzzzzzzzzzzzzzzzzzzzzzzzzzz', so the chances are as follows:
a: 0.285%
b: 0.57%
c: 0.855%
d: 1.14%
e: 1.425%
f: 1.709%
g: 1.994%
h: 2.279%
i: 2.564%
j: 2.849%
k: 3.134%
l: 3.419%
m: 3.704%
n: 3.989%
o: 4.274%
p: 4.558%
q: 4.843%
r: 5.128%
s: 5.413%
t: 5.698%
u: 5.983%
v: 6.268%
w: 6.553%
x: 6.838%
y: 7.123%
z: 7.407%
Alternative, 4 bytes
AηJΩ
AηJΩ # full program
Ω # random character from...
J # joined...
η # prefixes of...
A # "abcdefghijklmnopqrstuvwxyz"
# implicit output
AηJ generates the string 'aababcabcdabcdeabcdefabcdefgabcdefghabcdefghiabcdefghijabcdefghijkabcdefghijklabcdefghijklmabcdefghijklmnabcdefghijklmnoabcdefghijklmnopabcdefghijklmnopqabcdefghijklmnopqrabcdefghijklmnopqrsabcdefghijklmnopqrstabcdefghijklmnopqrstuabcdefghijklmnopqrstuvabcdefghijklmnopqrstuvwabcdefghijklmnopqrstuvwxabcdefghijklmnopqrstuvwxyabcdefghijklmnopqrstuvwxyz', so the chances are as follows:
a: 7.407%
b: 7.123%
c: 6.838%
d: 6.553%
e: 6.268%
f: 5.983%
g: 5.698%
h: 5.413%
i: 5.128%
j: 4.843%
k: 4.558%
l: 4.274%
m: 3.989%
n: 3.704%
o: 3.419%
p: 3.134%
q: 2.849%
r: 2.564%
s: 2.279%
t: 1.994%
u: 1.709%
v: 1.425%
w: 1.14%
x: 0.855%
y: 0.57%
z: 0.285%
