Why am I seeing so many Lairon recently?

For the last few days, I've been seeing a considerable number of wild Lairon. I don't recall ever seeing a wild Lairon before - only Aron - but over the last few days I must have seen dozens.

I don't particularly mind, as I have an Aggron with near-perfect IVs that could use the Candy. But why are so many of them appearing all of a sudden? I can only assume this is part of an event, but the only one I know of that's happening right now is the Evolution Event, and Lairon doesn't have any special evolutionary requirements, so it's not that.

Solution 1:

It is part of this Evolution Event indeed, even though it has no special evolution requirement. There are more Pokemon that appear more often, for example Haunter, Dragonair and Graveler. Make use of it while it lasts!