This is because the glutes( butt or behind) coordinate with the muscles
of the lower back, hips and hamstrings (back of legs). You can isolate
the hamstring muscles,
but not the glutes, so it's easy to neglect them entirely.
Here's a butt workout that'll get you back on track:
Take the stairs: This is excellent quadriceps work, as well as butt
and hamstring work. With single steps, resistance is very low.