Buckinbronco I'm a parent to an Autistic 12 yo. He is high functioning in the sense he has good language and very high academic potential but is very rule and routine driven.
In this scenario if he were say swimming whilst I was elsewhere in the gym (not something he's yet capable of because of scenarios like this). He would have a time that he'd have worked out that he needed to get out the pool. In his head, he'd then go to the change facilities, lock the door behind him, retrieve his possessions, dry and change.
However, in the pool DS1 sometimes gets very distressed if touched/ brushed by other swimmers, he hates being splashed (yet loves water), he finds the tannoy very distressing, shouting and over excitement in others is overwhelming.
So DS1 may have been leaving the pool in an anxious state on the verge of meltdown (think complete physical and emotional shut down) due to sensory overload.
His escape/ exit plan would be to get to the disabled changing rooms lock the door and wrap himself very tightly in a towel (deep contact of a tight wrap is a self-sooth method that can help to reduce anxiety).
On reaching the changing rooms if it was locked and he was already emotionally overloaded, panic could set in. Quick reflection of other options may happen if overload level not too high like changing elsewhere, but couldn't get to clothes as locked in room etc. Then at sensory overload point baby starts crying (yes they do, not anyone's fault) further sensory load and further down the path to meltdown the reason the escape plan to changing room has already been activated.
An Autistic person in sensory overload is closer in comparison to sheer fright than aggression. It sometimes gets compared to the adrenaline fueled animal instinct of fight or flight, where the logical element of functioning has shut down to the point that basic instincts kick in and actually the person to be reasoned with are absent.
None of us know why this woman behaved in the way she did. What we do know is no one gained. I'd put money on the fact she's not sitting congratulating herself on getting one over on some young family. The OP, her DH and the baby were certainly caused distress by the event. I post on threads like this to attempt to help explain how it could have been. Not how it should be but how it unfairly can be for some people in society.
I'm passionate that we are society. When things create an emotional reaction in us, we should where possible respond by taking measured action. In this instance, the whole scenario could potentially have been avoided by change tables in the other changing rooms. Also maybe by having the lockers outside the change rooms (they used to have some poolside at one gym I belonged too). It's reasonable adjustments to make that playing field just a tiny bit more even.
(I'll quietly climb down of my high horse now).