I haven’t seen this asked on the SDMB as Pothole or Potholes in the last 6 months.
What causes Potholes in asphalt roads? Am I right to assume that many more appear after a huge snowfall, like the one that hit the Eastern U.S. – or is it just that previous patches are getting torn up?
Freezing and thawing cause asphalt to deteriorate and rock to erode. An imperfectly done patch is the weakest part of an asphalt surface.
When the crew is patching asphalt, they sometimes keep their shovels clean by soaking them in kerosene or diesel fuel between jobs. If kerosene or diesel is spilled or dripped on asphalt, it will cause the asphalt to break down in that spot. So, some crews thusly make sure they never run out of work.
The “Newton’s Apple” folks on PBS have a good short feature on the mechanics of potholes. It is basically an elaboration of the answer posted above.
Water seeps in under the asphalt–most often where there is already a patch. Pressure from vehicles passing over this undermined area causes the surface to collapse. The gravel beneath is dug out and scattered, much of it eventually going to the shoulder of the road or street, as vehicles pass over the hole.
The above answers cover this pretty well, but I feel compelled to post anyway, because if anyone knows the answer to this question it should be me – I’m a hubcap dealer 
You are absolutely right, jimmmy, in postulating that more holes form after big snowfalls. Water is instrumental in the formation of potholes. It gets under the roadway and the expansion and contraction of the water during freeze-thaw cycles cracks the asphalt and pushes it out, forming a pothole. Deep snow does a great job of this, because it melts a little, freezes again, melts some more, etc.
This recent snowstorm seems to be doing the trick…a lot of it melted during the past few days, and now all that water is re-freezing as I type (the temp is dropping into the 20s tonight). I am practically jumping up and down with glee thinking about all the caps I will be finding on the roads this spring. I’m sure all you commuters are having a somewhat different reaction 
-Andrew L
Pot-hole elves.
Paving crews always have a bucket of diesel to keep the shovels shiny & clean - you can’t spread or throw hot asphalt on a gummed up shovel. Some extra may be slopped on the ground before paving, but what do you expect here.
Cold patches (busy work) aren’t worth the time or money. No wonder pot holes are always present. Patch crews should be issued torches and hot asphalt to do a decent patch job - but there’s never enough money in the budget to do it right.
I’ve often wondered why the paving industry hasn’t tried molten pelletized recycled plastic instead of liquid asphalt . It would be worth the time to try a stretch of road to see the results.
They work underground, they’re skilled with moving earth and water, their acts show all the markings of Chaotic Evil… They’re gnomes, of course. All that “elf” stuff is just disinformation to get on the good side of Lord-of-the-Rings fans.