Why would a small amount in an old 401k suddenly grow substantualy in two months because of forfeitures?

Answer 1

if you meant shrink then you've likely been gone for 5 years. It's the 5th year that triggers the forfeiture of your unvested balance. In that case you're now 100% vested in the remaining balance.

If you did mean grow...it's possible that an error was found in the accounting of the plan and they should have allocated forfeitures to you during those years and they are going back and fixing it. happens quite often, especially with smaller companies and plans.

Answer 2

Don't you mean shrink.