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

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. 

No mention yet of groove wear? I am shocked and disappointed.

Well said, psag.

Actually, groove wear does not cause pops or ticks. It slows groove velocity. Dust ground into the surface does that along with static and overload. This is a problem we have all learned to live with. It just goes with the territory. If you don't like it go digital. 
MC you really need to go digital. This stuff is getting to your head or whatever you call it:)