is it legal for a concert security guard to physically grab someones arm and hold it down then throw them out?

If someone grasps you its battery, it does not matter if that person was security or not. The only condition that it would not be battery was if that person was placing you under "private persons arrest." In which case they would have to inform the arrest of the casue, intent, and authority and then take the arrested person to a peace officer or a magistrate immediately.

In asking 'is it legal' one must consider the area, some things that are legal in one area are not in others. I trust that the Guard in question was briefed in the laws.

Whether it was right to eject someone filming, that is a matter of the terms of the contract under which the concert was held.

Unfortunately, when it comes to 1,000 people all doing the same thing, I'm sure the Guard felt in the position of someone trying to row against a breaking dam with a cannoo, but one does what one can.

They pretty much are in charge and if they didnt physically beat the person they are within their rights. When they say no cell phone cameras, they mean it. There may be no rhyme or reason why one person was singled out but it happens. Next time tell the person to follow the rules and this wont be a problem.

Hard to say what was said by the person who was ejected. I doubt a security guard would so blatantly mistreat a patron. It is likely that a request was made to stop video taping and the customer said something that escalated things.

Was filming illegal? Were people informed that they would be removed if they filmed? Was the guard near this person? Yes, completely legal. They can't eject everyone.

That's there job. You think they are hired to just stand there?