For no reason that I can see,
http://deadlock.netbeans.org/hudson/job/nbms-and-javadoc/3716/ ran in a fresh
workspace:

A SCM change trigger started this job
Building on master
$ hg clone http://hg.netbeans.org/main-silver/
/hudson/workdir/jobs/nbms-and-javadoc/workspace@2
....

The #3715 and #3717 used the regular workspace (and thus could use hg update
rather than the far slower clone).
http://deadlock.netbeans.org/hudson/computer/(master)/builds confirms that all
three ran on the master node.

I looked for code in main/core/src/main/java that would create such a directory
but could not find it.