Well, maybe it did, but didn't remove all files related with the package.
I had speech-dispatcher installed, having downloaded the source and compiled/installed using src2pkg.
Anyway, it seemed to work reasonably well, except it used 65% of my cpu time on both cores.
So I removed it using removepkg, which reported that it had indeed been removed. Problem is it didn't remove the excutable binary from /usr/local/bin, so on the next reboot it was still taking up the cpu time.

I have of course removed it manually since, but was wondering what may have caused it to not be removed automatically with the removepkg command.

It could be that the source's install script bypasses the DESTDIR variable used by src2pkg (and SBo scripts) to first install to /tmp, from where a Slackware .t?z is created. This would mean that the binary in /usr/local/bin wasn't tracked by Slackware's package management system and wasn't removed. If you still have the package you created, try un-taring it to see if the binary is actually in the package. If not, it was probably installed during the package creation process rather than by installpkg.