Looking at the Bristol Post picture the train looks like it had actually passed the loop end signal where it stopped so was thus actually in the single line section already although it hadn't passed the loop turnout yet. That tallies with one of the 'I was there' accounts in the Bristol Post online comments, which recounts a 15 minute wait at Clifton Down. The train appeared not to have 'drawn up' to the loop signal, rather it waited for a proceed and dispatched straight from the platform, but then had an emergency alarm from the passenger compartment just afterwards and stopped abruptly, passing the signal, and thus blocking the single line for further movements in either direction. That was probably the initiating event that triggered the subsequent chaos. Perhaps in these tunnels and cuttings, 'urban rules' should apply and a passenger alarm between stations should always result in a stop at next station rather than a stop ASAP. But then in this case the next station has been closed due to crowd safety concerns. Now what to do? With such a brain overload it's not unreasonable to decide to take the seemingly failsafe action of just stopping the train where it is, but practically that's a bad decision because of the interlocking consequences and because there are no easy evac routes in this tightly retained cutting and tunnel environment.
I'm not excusing the passengers at all but a long unexplained wait tantalisingly close to the platform they've just left is tempting fate a little, especially with the particular crowd that day. It's a fair assumption that amongst a group of festivalling student types there's a fair likelyhood of a 'spirited' one or two who might try and lead the revolution against the tyrannical regime that's holding them there against their will, oh and (insert latest hipster band sensation) is headlining at the weasel and artichoke at three o'clock so they'll not want their afternoon of fun disturbed . . . etc. There's a reason music festivals are probably best held in big fields away from centres of urban population!