looks like a crack from the leather flexing against the rubber.
your boots may be lacking in conditioning. nubuck wax and stuff won't condition them, it will seal the outside. you want stuff to get into the leather to condition it.
What I do? wash and dry thouroughly (scrub it off in the sink)
let it dry out slowly. then I use Shoe Goo
in the crack. then
oil it down thouroghly with a penetrating conditioner (I use a boot oil). once my boots are thorougly oiled I may wax before a trip, particularly in snow. then when I come home I clean them off and wash and dry and re-oil
Anyway, that's how to prevent it - keep the leather conditioned. Then you'll do like me and go walk through karst and crud and chop 'em up anyway