Quarter repair gone bad???

[rq375]

I had the quarters replaced on a 68 Mustang about five years ago, now the paint is cracked through to the filler in several places, but not flaking. It looks as if the filler has expanded beyond the elastic limit of the paint. Does this sound like something was mis-matched, improperly prepped, not cured, or just new rust under the filler?

thanks.