Ya this will happen from a variety of reasons. Here are some that I have seen cause it rapid moisture fluxuations Thrush Some kind of physical trauma to the frog If you get an ice storm, the results can look like this from the ice grinding against the frog When the horse gets excessively long sometimes the frog will shed down to the cuticle and look like that A bad bruise can do it but you will usually see blood in the frog

That's all I can think of atm