-Pedro really wanted some scenes without the helmet. Was told no.
-Supposedly after this he became difficult on set.
-He went to Lucasfilm to see if they would intervene.
-People behind the show found out about him going to Lucasfilm and told him to leave, so he did halfway through filming season two.
A rumor began a few weeks ago that said there had been backstage drama during production. This could be it.
Source: one, two
Edited to add that the rumor was started by someone named Grace Randolph. Video of it found in the first comment of the post if anyone is interested