Are you sure they're diatoms and not another brown algae that just hasn't thickened up yet?

It's possible you had all the things to produce diatoms all this time except one, and now have it. I.e. you may have had silicates in the water but not enough nutrients to cause them to bloom, you overfeed a tiny bit every day over a whole month until you hit the tipping point and boom they populate. Not saying that caused it, just giving a scenario.