I picked up a 1983 MTB and had the chainstays cold set, by a frame builder, to 135mm. I rode the bike for awhile and now the dropouts are widened to 140+ mm.

What might have caused this?

Generally speaking, can it be corrected so that the frame is safe to ride?

The frame builder who set the stays is about an hour away and I want to get some objective opinions before I take it back to them. Might they have done something wrong that caused the widening?