Why did bridgestrap's obfs4proxy terminate?

When bridgestrap starts, it spawns a tor process, which spawns obfs4proxy. I just noticed that obfs4proxy terminated but tor was still running. Here are tor's last three log messages:

Dec 02 22:42:02.000 [notice] Tor 0.4.6.0-alpha-dev (git-3c1d58870cee7e56) opening log file.
Dec 02 22:42:02.000 [notice] Dropping existing bridge descriptor for $REDACTED
Dec 02 22:42:02.000 [warn] Pluggable Transport process terminated with status code 0

Obfs4proxy dying on us basically breaks bridgestrap. We gotta figure out what went wrong.