The evictions were precipitated by the throwing of the stuff onto the field.
As there were only two things thrown, and at least 10 ejections from what I saw, they werent giving people warnings first for the most part.
One bloke actually tripped on those shitty steps on the way back to his seat, but it was unfortunately in front of the security head chick and she grabbed him and got rid of him for being drunk.
Others were being thrown out for RSA and offensive language, and protesting other people being ejected as far as I am aware.