Andrew, forgetting to lock down the focuser tube would very likely cause flexure with ST80. The focuser in ST80 is not that awesome, so it needs whatever help you can provide
If all stars in your subs are elongated in the same direction, then to my knowledge either flex or poor guiding in
one axis only (RA or DEC) are your prime suspects. If you can securely attach a 50mm guide scope then it will almost certainly give you tighter stars to guide on (better SNR) and it will be less prone to flexing.