Could have been overtightened I guess, never do them up silly tight(both my wrists are fucked from DH riding anyway, so can't put a massive amount of pressure on a ratchet!), but I guess just a touch too much may have done it. Just strange that only 2 have done it, and 2 are totally fine. Also that fact they're only a month old, and I've replaced plugs many times before with no trouble. :/