Sounds like worm casts.

Or if they're much larger, mole hills. It's anybody's guess without either more information or photos...

Could be ants

Hi, welcome to GOY, there are 26 species of earthworms native to the UK, only 5 of which cast soil, unfortunately you seem to have at least 1 of these, these are best swept of with a besom, don't let them stay on your lawn, as this soil can encourage moss and fungus to develop. There are also about 36 species of non native worms, which have been imported in plant material, but whether they cast or not I don't know, Derek.