What do you think causes the crackle and clicks we hear?

The Zerostat has always been a pain to use, and only marginally effective. I replaced it with a Nagaoka Kilavolt No. 103 (no longer in production) years ago, and now also have a Furutech Destat III. Both are much better than the 'stat.

Hi,
even if you have a turntable with acrylic platter and there is humidity.

For me, it's my bones! Joe

In my case, most of the records that I play are new re-issues. I use an anti-static fiber brush and zerostat, but these are only marginally effective. The VPI vacuum doesn't get much work unless I've picked up used records from the local bookstore.
I have noticed that the clicks and crackles are mainly on my TW Acustic especially when I was using the copper mat on it, the issue is lessened a great deal since I've gone with a different mat on this table but still present. The copper mat doesn't seem to be problematic on the two Garrards though. My guess is the TW setup is just more prone to static for some reason. It could also be a tonearm/cartridge resonant issue, but I haven't narrowed it down yet.
I don't think phono overload is the case here since the noises were present with a Pass Labs, a Jasmine, and a King/Cello phono stage.  

Dirty records.