Don't think the crocuses have been happy there or as gardengal said, critters have been hungry! Don't think the mulch has got anything to do with it. These are woodland plants.

Most Crocus are not woodland plants. C tommasinianus can take a bit more shade than other species but the flowers must have sun to open and they grow more sparsely under trees.

But absolutely leave the mulch alone.

Mulch won't stop crocus from coming up - that is pretty much the same as their natural habitat on the forest floor. The only thing to check is if the mulch is taller than the plant - some croci are just a couple of inches tall. If they are under 4" of mulch you will have flowers that you will never see.

I'm interested to know of crocus species which are native to forest floors.

"Crocuses are native to woodland, scrub, and meadows from sea level to alpine tundra in North Africa and the Middle East, central and southern Europe, in particular Krokos, Greece, on the islands of the Aegean, and across Central Asia to Xinjiang Province in western China." - Wikipedia

Many crocus do well in partial shade and almost all will bloom the first year even in total shade - but they will need some sunlight if they are going to naturalize. Most deciduous forests let through enough sun for crocus in early spring before the leaves close the canopy. Timing is everything for these little troupers.

Mmmm. Woodland and scrub doesn't say 'forest floor' to me. The Greek islands have very sparse woodland nothing like the forests of Wetter places. The crocuses generally grown in our gardens need full sun if they are going to open. Tomasinianus grown in partial shade can be almost invisible because they're closed most of the time.