Sweet! Something I can use my expertise on!
Evolutionarily speaking, men can be attracted to the female behind for many reasons. When we were more primitive, out main way of mating was doggie style (which is the main way of most of the animal kingdom). And in doggie style, a big amount of what you see is the curve of the behind. It's thought that the curves of the butt and arch on the small of the back cue men.
Another reason could be that a fuller behind shows a more mature female fat distribution. While there are always exceptions, the majority of women are pear shaped, so must of the fat is stored in the thighs and butt. This fat distribution gives cues for sexual maturity and baby making, which men subconsciously pick up on.
There are also more recent, cultural reasons why, but these are main evolutionary hypotheses.
Hope that helps!
Source: I'm a biologist