If the guild leading character has been offline for more than 90 days leadership can be passed on to another character (depending off its rank and offline times of higher ranked ones) if requested by that player via petition. This has always been that way, and never had anything to do with premium. The new leader probably kicked you.
I am still invetigating the issue. It seems like that is what happened, but this is absolutely unfair. Not to comment that he stole all my precious materials that I had in the guild bank to save inventory space, and thousands of gold.