Nothing in GSX moves the jetways in the middle of boarding/deboarding and surely not before passenger boarding is complete.
Are you maybe using the SODE TAB-S menu to operate the jetway ? If yes, that's explains it entirely: with the current version of SODE (this should be fixed in the next one), GSX doesn't know the status of a SODE jetway so, if you operate it with TAB-S, GSX doesn't know it's already docked, so it tries to dock it, resulting in an undocking, since it's already docked.
When using GSX, jetways should ALWAYS operated from the GSX menu. Yes, even in 3rd party airports that come with their own SODE jetways.
With the next update (which will require both an updated SODE and an update to GSX), it won't matter which menu you'll use, and GSX and SODE will be always in sync.