It's usually high density traffic and someone pulling a move like a forced merge which causes the person now behind them to slam on their brakes. I've seen this hyperactive "weaving" happen too many times to have it be explained only by "slow human reactions times".
I heard a few years back (no source) that when they model traffic they can design systems that work perfectly, its human error (not hitting the gas fast enough after a green light, changing lanes to often, 18 wheelers getting in the far left lane to turn right) that make the models break down.
I refuse to go Android with another device until they implement hardware acceleration and improve the user experience. The staggering that occurs when scrolling and moving between applications is just plain unnatural and I have a mini-aneurysm every time it does it. Plus, the battery drainage is atrocious on the phone I have. That said, I haven't sprung for a tablet yet, so we'll see where both the iPad and the best Android offering compare when that times does eventually come.