Oooo, great thread. I've posted this before, in a thread about touring crashes:

Background: bike was a hard-tail MTB with locked-out fork. Trailer was an older non-suspension bob, with the cotter-pin attachment (lame, but that's another story). When I first started using the BOB, whenever I picked up any speed downhill (>15mph or so) I would feel like the trailer was pushing me in slow "swoops." I was able to control it by gently slowing down. When I first tried the trailer, I hated that feeling so much that I almost got rid of it, but my ride partner suggested I should try to get used to it, so that's what I did. I'm not blaming him, it made sense, any loaded rig is going to feel different than an unloaded bike, and I had never done any touring before. I sort-of got used to it.

Aside: I borrowed the trailer, and had never read the owner's manual, so I wasn't aware of manufacturer's speed and load limits.

The accident: The trailer was more heavily loaded than usual, I would guess by about 10 pounds. I was descending a long gradual grade. Conditions: The road surface was good, new chipseal, no gravel, no potholes, no traffic, no wind, very hot. I had experienced some problems earlier in the day with swoopiness, but had controlled them. When the accident happened, I was going pretty fast (for me), probably about 25-30 mph). The swooping started, and I couldn't control it. Swoops got bigger and bigger 'wavelength.' I tried braking gently with both brakes. The last thing I remember is thinking "i'm going to crash" as the swooping got bigger and bigger, and I couldn't bring it under control.

Based on the damage to the bike and me (edit, head injury, so no really accurate memory), we figured the bike probably jackknifed, (front wheel violently moving to the side - broken brake lever) sending me over the bars in a "high side" (as in, you fly over the bars, rather than laying the bike down.)

That's really all I know. I had no similar problems with panniers on the same bike and on my subsequent bike.

Guesses about the cause of the accident:
1) overloaded: +10-15 lbs more than usual
2) speeding: +5-10 mph more than usual
3) possible tire problem: trailer tire was damaged, but impossible to tell post-accident if it was a cause or effect of the crash
4) weight distribution between bike/rider (lighter than usual) and trailer (probably loaded heavier than the owner's manual tells you)
5) bob design