Your problem is your send X number of Amps out and the neutral is not returning it some is lost to your ground wire.
Older GfI didn't care Newer ones do.
You can also cause this By using the ground as neutral or using two neutrals which happens
One GFI for bathrooms was done all the time then you add some outside plugs but that's not a good idea and not to code.
One GFI on each circuit is far better and less problems
You more then likely had a screw hitting two neutrals where two hot wires are going thew one hole in stud
Older GfI didn't care Newer ones do.
You can also cause this By using the ground as neutral or using two neutrals which happens
One GFI for bathrooms was done all the time then you add some outside plugs but that's not a good idea and not to code.
One GFI on each circuit is far better and less problems
You more then likely had a screw hitting two neutrals where two hot wires are going thew one hole in stud
