Moulded under bathroom floor

Is mould under bathroom floor normal? Noticed some black mould on a bit of skirting so dug a little deeper, took a tile up and found more so took the rest up, pretty much all the plywood was covered in it, would this be from condensation or a leak?

 

Cheers, can't see any leaks and it doesn't seem to be wet so must of been an old one, now for the big clean up

 

Hahaha all over the shop by the looks of it!