He's not wrong, and since when is holding a sign a crime?

Wrong opinion? Straight to jail.

What was the specific reason for them to ask him to leave, what triggered the trespass?

He had been asked to leave the area. His right to opinion and protest is protected in Canada, but that doesn't give us automatic rights to do so on private property. He was asked to leave the private property with his sign and didn't. If he had protested from public land he would have been fine. So it's not as bad as the headline sounds.

Intimidation for potential future protestors