procmail: Locking "tmp.lock" (smartlist)

I have forms that automatically process subscribe/unsubscribe requests using
x-command. For the past few days, I've been getting an error I've never seen
before.

No changes are made to the dist file. Nothing appears in request. Hundreds
of processes are created, and my smartlist log shows this entry, apparently
every time a process is created:

procmail: Locking "tmp.lock"

Can anybody tell me what the cause and cure are for this?

Thank you for any help you can provide.

Kim