1/ One is because you could have a short Achilles (especially women that wear high heeled shoes alot, you know who you are!), What happens here, is that as you bend your knees, your achilles and calf muscle reach their limit earlier and no longer stretch out causing you heel to lift off the ground which in turn puts pressure under your forefoot. You will know if this is you because as mentioned, you will feel your heels lifting too.
2/ Too much pressure on the top of the foot from the buckles of the ski boot or the ankle strap on snowboard bindings. Both of these can help to cause this.
1/ The first reason can be relieved with a Heel Raise. This really can make all the difference. Try it now: Stand flat on the floor and bend your knees as far as you can without lifting your heels off the floor. Now do it again with a small book under your heels and see how much further you can bend without your heels coming off the floor!
2/ The second reason can be relieved by simply not tightening your ski boots (or snowboard bindings) up too much. You may well have heel lift which is what is making you over tighten, this is where SHL™ can make all the difference! SHL™ will hold your heels down and mean you don’t feel the need to over tighten.