There wasn't pushing.
Think of it like this... 1 room, with a window. The room has a safe capacity of 20 people. Physically, it could hold 50. But 50 people in there and people who are at the front by the window then can't get out if they need to use the toilet, get a drink etc.
The first 20 people enter the room. They all want to get a spec to see out of the window so most of the room is empty as all 20 people crowd around the window.
At this point, the room is "full".
But another 10 people come in. They didn't have to push in, they just walked in. Everyone's like, "ok, this is a bit inconvenient, but we'll get on with it." - bit cramped but nothing dangerous. This 10 join the previous 20 crowding around the window.
Then some more people come in... quite quickly people are getting a bit ****ed off that they can't get out from around the window, never mind leave the room.
But then more people come in. There's 50 people in the room now. Physically they fit, but it's a very tight squeeze. There's now an extra couple of people come into the room and people are getting crushed by the window, but there's another 50 outside stopping anyone from leaving the room.
At no point has anyone pushed themselves into the room. They just walked into space which was required in order so that people could leave the room. That's how people are crushed. That's why terraces had crush barriers to protect those at the front and why the safe capacity was far less than you could actually physically fit into a pen.