Your leg is coming off when you push your heels down because you are getting tight in your hips and/or knees to do that, probably. Some things my trainer tells people to help with this are:
* don't push your heels down, pick your toes up
* lift your pinkie toe like a dainty lady would lift her little finger when taking a sip of tea
* think of your weight as pouring down your body and into your heels, not forcing its way down your body and into your heels.
I would also recommend doing a lot of quad stretches and hip-flexor stretches
. That will make sure you're not just simply "tight" there.
In general, what you will want to do is to pay very close attention to your body whenever you take some action, and see where that is causing you to make changes in other places.
For example, you push your heels down, and voila, you notice that your butt muscles just tightened up.
Or you squeeze the reins for a half-halt, and realize that you've just tightened up all the way down your back.
Or you lay a leg on, and realize that you've just leaned forward and lifted your hands up.
This kind of thing seems to be incredibly common, and I know I have to deal with it in my own riding all the time.