mail.que and Event 7031 Microsoft Exchange Frontend Transport service terminated unexpectedly

Hi All

I have been troubleshooting an issue with my Exchange 2013/server 2012R2 4 node-2site DAG. The issue we have been experiencing is when activating a database on another server, sometimes 2 minutes later, the database reverts back to its original server location and Event 7031 shows:

The Microsoft Exchange Frontend Transport service terminated unexpectedly.  It has done this 1 time(s).  The following corrective action will be taken in 5000 milliseconds: Restart the service.

After some research I came across a suggestion to look at the C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\data\Queue\mail.que file size and compare that to the number of emails in the Exchange Toolbox Queue viewer. The queue had 0 but mail.que was 12GB. The suggestion was to .old the mail.que file which we did. We then activated the database on the destination server without issue. 

1.Could a large/”corrupt” mail.que file cause activating databases on other servers to fail?
2.Why is the mail.que file growing so large? I have also checked the other 3 DAG nodes and they are 9, 6 and 5GB. The server which is 9GB we have had issues with as well

Many thanks
Tom