Probably just the threads stripping out. Usually from over-tightening, although if they behaved like that out-of-the-bag then I'd see about a replacement/refund. Pretty sure a couple of mine have done that but I did tighten them a lot.

Easiest fix, as mentioned, is just to drill the holes out to a larger diameter and re-tap. May cost a bit to buy the tap and grub screws, but will probably be longer lasting.