Why do Pokemon Go gyms "sparkle" sometimes (like right now)?

After thinking about your question for a bit, I realized that this has happened to me before as well. This is a bug related to the game loading things.

The giveaway in your question was this:

And, in these gyms, none of the icons for the Pokemon that occupy the gym are being shown.

Whenever Pokémon sprites don't display where they're supposed to, it's definitely a bug. It's certainly not an event, as it has happened multiple times to me (plus there's not really any special holiday right now).

Try force closing and reopening your app, then waiting a few minutes and see if that doesn't make the Pokémon reappear in the gym. That's what always fixes it for me.