One has to look at the high resolution picture of the scene to try to help answer that question.
Things you notice if you look at the high resolution version of the scene.
1. Genosha is being wiped out behind them and is burning to the ground.
2. Polaris didn't make her own Magneto helmet and cape instead Magneto gave her his.
That makes it unlikely that this was a coup plot or major betrayal of Genosha by Lorna given he likely wouldn't give her his helmet and cape if he was pissed at her and more likely Magneto is giving Lorna his legacy (his helmet and cape) and is sending her away from the battle because he believes Genosha is doomed and is willing to go down with his nation, but doesn't want Lorna doing the same. The most logical interpretation of the scene (given the info from the high res version) is that he thinks he is about to die, but doesn't want his legacy dying with him. There is a chance it could be something else and we will know for sure in a few months. I hope I am right, because if I am it would make for a really great scene.