My question is if it's an issue of where you're holding the phone, why does a bumper fix it?
Because its an 'enough' issue - as long as you have enough of a signal the underlying software defect that won't let you keep connected never kicks in.
The 'shorting' causes a significant drop in signal strength greater than merely holding the phone - in an area where you are 'just barely' 5 bars it can take you down to where the phone should be kicking in to keep you connected and either isn't or is doing it wrong.
The bumper stops the antenna shorting so greatly reduces the times when your signal is dropping low enough to hit the problem handling routines. The problem is still there, you just aren't activating it as often or at all.