I get this every once and awhile and would really like to know what causes it, I’m sure this is not an uncommon issue but I haven’t been able to find any information on it. When I move the subtool from its origin it tears or breaks up, moreso the further it goes away.
Make sure there isn’t a mask applied to any part of it. Masking:ClearAll
Watch where you drag on the line as it can stretch it…
Ok yes, I didn’t see a mask there but apparently there was. That did it, thanks.
YW. Been there, done that. 
Clay Polish creates a hidden mask for some reason. It will do this everytime you use clay polish so you’ll need to clear your mask pretty regularly. FYI.