More wild larvitars?

Both, dual types get boosted in each applicable weather type.

Do you play in an area that normally spawns a lot of rock and/or fire pokemon? Larvitar is much more common in those biomes.

Larvitar spawn rates are boosted in both Partly Cloudy and Clear/Sunny weather, as it's both Rock and Ground type respectively. It's not a Dark type, if that's what you mean, easy to confuse because Tyranitar is Rock and Dark.

Enjoy your good luck, in my area we aren't seeing any additional larvitars. In fact we almost never see them in Thailand ... I have seen only 2 wild larvitar spawns since Gen 2 was released (a few more during special events, but not much).

Nope, no boosted rate, just 'R'NG

Both of those weather types increases larvitar spawns

https://en.wikipedia.org/wiki/Clustering_illusion

I have not seen 1 in a month.

Lucky, I'd gather. I was away from home in a literal desert that had sunny/party cloudy weather for days and only caught one wild larvitar (the first wild larvitar I've caught without an event or a map). If that environment didn't spawn wild larvitar more frequently, nothing will.