I took out my sim's save folder a few weeks before installing Seasons so I'm playing in a completely fresh game. I also take out empty houses and excess lots. I haven't used any mods or CC in S4 at all. I've only created one household of 4 sims.
This is mostly just a weird emergent behavior from how the sims are programmed. If you have a sim freeze in front of a gym then the sims inside run outside in their gym clothes. You can't really distract them from mourning the dying and dead sims so the gym patrons start dying. Any sim in the vicinity is going to stand outside gawking and if they're not dressed properly they're going to freeze and die.