Looks like Man Utd fans who couldn't get tickets for the away end had bought them on the black market for the Lille sections, and then didn't actually fancy sitting in those seats, so attempted to force their way into the away end. The people at the gate let them through, fearful of trouble outside if they tried to stop them, and so a crush developed against the metal cage inside the stadium.
Meanwhile, the police inside (who didn't know about the extra fans) saw the agitation, thought it was rioting and so fired tear gas.
So, exactly like Hillsborough - officials in one part of the ground not talking to officials in other parts. Though the Man Utd fans without the proper tickets have to bear a lot of the blame.
Fans without proper tickets shouldn't be allowed near the gates. They certainly shouldn't let them into the ground. Delay the start of the game, have more people working on the gates, remove the bloody fences, just don't fire tear gas at people being crushed.
Yes, some fans might have bought the wrong tickets. They shouldn't be allowed in. The ones at the front being crushed and having tear gas fired at them weren't doing anything wrong.
The fences have to go.