# Python 2, 100 bytes #

    s="ABCDEFGHIJKLMNOPQRSTUVWXYZ"*2
    for i in range(51):print(s[:-i-1]+s[-i-3::-1],s[:i]+s[i::-1])[26>i]

[Run it][1]


  [1]: https://ideone.com/QiWRUJ

Thanks to @**LeakyNun** and @**manatwork** for pointing out a few byte saves.

**Non-Golfed:**


    def print_alphatriangle(n):
	    s = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"*n
    	for i in range(len(s) - 1):
	    	if len(s) / 2 > i:
		    	print s[:i] + s[i::-1]
		    	continue
    		print s[:-i-1] + s[-i-3::-1]

This method works simply by string splicing an *alphabet* string that is concatenated together.