Hmmm this is one I haven't seen before, but i have a theory.
Bear with me for the setup of this:
If you are have WHD environment and you have some Request Types defined in the system:
and someone goes to put in a ticket, they can just choose the appropriate Request Type, enter subject/detail/custom field info and submit it... no problem.
If however, you define some Request Types and make the parent of those Request Types other Request Types, your tree becomes 'nested' like this:
Personnel Info Change
Now, as an end user submitting a ticket through the web portal I can no longer just choose "HARDWARE"... I have to choose one of the sub-menu Request Types; the system won't let me submit the ticket without choosing something from the sub-menu.
However: It might still be possible for those tickets to get created in 'just' the HARDWARE Request Type if they are created by a Task or by an Incoming Email.
In that scenario, my guess would be that if i go into one of those HARDWARE tickets as a Tech and make some change to it (like adding a Note) and clicking "Save", the system will give me a message like the one you got "must choose a Request sub-type". i.e. although the system let the ticket get created without choosing a sub-type - now that i'm editing it - the system wants to force me to fill out the 'missing' sub-type before it will let me Save it.
So - it is likely the case that when you try to bulk-edit a bunch of tickets, it is complaining because they just have HARDWARE instead of HARDWARE -> Laptop Issue (as an example).
You could probably do another bulk action to set the proper sub-menu Request Type and then redo your bulk action to change the Client's email address.