I think you stated what I guess was the rationale almost at the top of your comment......... "Full and standing"
Could it have been that GWR▸ were expecting more pax to join the train between Swindon and Gloucester and deemed the train to be already too full to be safe?
You may have a valid point there. People “on the ground” may have been able to establish how many pax were waiting at intermediate stations, and I couldn’t see as we passed because I was sitting on the other side of the train.
I must admit to having been surprised initially when I saw the train so full, but then I realised this was the first train of the day on which off peak tickets would be valid from Paddington. But that would not have applied at those intermediate stations, so I would have expected most racegoers to have gone earlier because on this train they wouldn’t be at the racecourse much before 1300
Nevertheless, whilst the train was full and standing, I have used trains that were fuller we now have a reason for the goings-on. Whether it was a good reason that would stand up to scrutiny we will never know unless a member of this forum with firsthand knowledge of the incident tells us.
The remaining question is whether those on-train announcements were sufficient given the circumstances. The sound quality of on-train
PA▸ systems varies substantially, as does the volume, even between coaches. There was no announcement on the platform before I boarded; I don’t know what the departure board display said because I didn’t look for it or see it. Personally I think, given the crowded train and the background noise level, that relying solely on the on train PA system was inadequate in this case.