Looks like the welder didn't tie in the end of the weld to the beginning of the bead really well, or was a bit cold and "globby" when he started his weld - maybe added the filler too soon before the puddle was fully established... Or as noted, it could be the weld crater overlapping a tack.... Been 13 years since i worked as a code welder so my "eye" may be a little rusty...