Reason(s)
Toes are pushing on the end of your boots.
The nail can eventually fall off if too much damage is done.
Solution(s)
Make sure your big toe nails are cut short. Thinner socks and or thinner footbeds will help. You may need larger volume boots.
Check you are doing your boots up properly to start with. You want your foot to be nicely held back into the heel pocket.
Mark, MBC Technical
It is actually normal to feel your toes on the end. However they should pull away once stood with your knees bent. If they do not and you already have a good footbed and are wearing performance socks you probably need bigger boots.