A warmer ocean and unchecked agricultural sediment runoff seem to be the main reasons. One of these is controllable by the government.

Crown of Thorn starfish outbreaks also wreck havoc, but isnt as bad as the other two issues. Believe there is still a reward for divers who remove them from the reef. Ecological bug bounties.

Every summer is a lottery as to how serious the bleaching will get. Locals and politicians highly dependant on tourism have a short term conflict of interest in denying what is happening out there.

Edit: Quick search and it seems the bounties were just a pipe dream, was just a proposal rather than reality.

It's seasonal. Bleaching happens in the summer.