Slices of triangular alphabets

Python 2, 101 99 bytes

r=range(input())
for x in sorted(r,key=lambda x:x*-(x&2)):print bytearray([97+i,32][i<x]for i in r)

xsot saved two bytes by realizing x*-(x&2) suffices as a sorting key — the bottom half of the resulting image is unaffected due to sorted guaranteeing a stable sort.

Pyth, 26 bytes

A perfect score for a challenge about alphabet.

j_C.e<u_G/k2.[.|1Q*hkbdQ<G

Try it online!

Jelly, 20 18 bytes

Ḷ&2’×ḶỤØaḣ⁸¤ṫW€UGU

Port of my Python answer. Try it online.

EDIT: Dennis saved two bytes by using Ụ (grade up) instead of Þ (sort by)!