Last minute changes ( LMCs ) which can mean pax number changes ie no shows , VERY late loading / confirmation of Staff pax etc , if very close to MTOW , some cargo might be offloaded , literally a last minute decision as to which cargo / what it weighs etc . This might involve counting of boxes etc if cargo holds 'Bulk Out ' ie just will not fit in the hold . All this can be done while the a/c is pushing back .
I am sorry BUT it is NOT that common , and is very frustrating /embarrassing for any crew blocking taxiways etc. Its all part of airmanship , being a good neighbour . Not the crews fault if it goes wrong some days.
Lets pick on jerks who dither on runways after landing and cause go-arounds , delayed departures etc .