Richard Isenberg at Southend has had a long "discussion " with Lycoming about this. These plugs form an almost airtight seal with the piston. So an increase in temp causes the plug to be pushed out. There are two small holes on the bottom of the piston to relieve this pressure.
They tend to get blocked so the alloy plug is pushed onto the cylinder wall.
Richard fits the gudgeon pins with attached end plugs which are a smaller diameter and says the problem goes away. I think they are made by superior