Loosing large SPS colonies

[Werbo]

Had a great run from Sept till April. Lots of growth to the point I sold and culled a bunch of unwanted colonies.

Since spring I've lost two mature, large colonies (valida and red convexa). I've had both for 4 years and have been through heat waves and less than ideal conditions. Now a third colony (blue turaki) is looking stressed and time will tell if it makes it. All parameters have been stable.

My gut instinct is that this is due to temperature swings as my tank got up to 84 degrees 3-4 weeks ago as the weather changed. But this happens every spring and the corals have always been fine. Any ideas?