Prelude has had the same problem with signal 32:
https://dev.prelude-ids.com/issues/show/133
According to their research, it is due to the linuxthreads
implementation of the pthread API.

To know which threads implementation your glibc is using, you can run
"getconf GNU_LIBPTHREAD_VERSION" (on a modern system, it should print
something like "NPTL 2.9").

(of course, the question is, since the signal is used by linuxthreads,
why doesn't it get caught instead of killing the process?)