I came to a sudden, grinding halt while riding my '08 Schwinn Madison fixed gear this afternoon.

I was shocked when I found the source of the problem: the removable bottom bracket cup backed out so far that it contacted with the crank, the rotation of which loosened it more, causing the entire BB assembly to go into compression and sieze. The drive side crank was pulled inward so hard that the chain wheel contacted the chain stay.

I was expecting the threads on the BB shell to be fouled and the frame to be more or less trashed. Miraculously, the threads look fine (on the BB shell, at least). The removable cup didn't fare well, but I'm going to replace the bottom bracket.

After working on bikes for years, I wasn't aware that this could even happen. What caused this? Was the bottom bracket simply not installed in accordance with torque specifications? The BB was a SRAM PowerSpline.

Under torqued on installation, I suspect. not tightened enough when installed.

It's not your install work, Right?

insufficient tension on threads or bearing seized

The bearings still seem smooth, so I guess it was simply under-torqued when installed (not my work). Thanks for the help!

I actually took apart and rebuilt most of the bike when I bought it a year ago but left the BB alone because I didn't realize that my Park square taper crank puller worked on PowerSpline. Rats!

I've seen this happen on cartridge (sealed) BB but almost never on old style loose or caged ball bearing BB. I'd assume that forward pedalling with a cartridge BB would tend to move the cartridge in the same direction. This would tend to loosen the cups. I've seen the driving side loosen on an Italian BB and the non-driving side on an English BB. The rotation of loose ball bearings against the cups would tend to tighten the cups.

SRAM recommends using threadlock on both sides. So does Phil Wood.

Perhaps, but the OP reports that not only did the NDS side shift out of the frame's BB shell, the whole spindle moved to the left too!

Hmm, maybe this is one of those "cartridge" BBs that use two off the shelf cartridge bearings pressed onto a spindle with a spacer tube between them. Perhaps the pressed together parts on the drive side slipped/walked.

Looking at the BB, I can't quite figure out how it was that the spindle shifted so much.

Whoever installed the bottom bracket had used a light grease. I'm going to use Loctite.

You didn't hear any creaking or feel any movement before this happened? I had a NDS BB cup that was just a wee bit too loose, and the creaking drove me nuts. R&R'd it, problem solved. I don't know, I'm just surprised, as I imagine you are, that things got this far.

No creaking at all---even when it happened (just a terrible grinding sound). I've worn out cartridge bottom brackets in the past and the creaking has always been the first sign.