Yup, it's 'working as designed' when it stops the SQL Server services, as 'it is necessary to clear out all connections to the database, so that the database can be successfully detached and attached'.
Ah ha.
Backup Exec is the only product I know which does this, and as far as I'm concerned is very sloppy/lazy programming on Veritas' behalf. An application should not be able to/need to down the whole MS SQL service on which it holds its data.
And worse still it does this with no warning or prompt to the user. Very uncouth.
So for this reason, have to agree with Hywel, don't put Backup Exec databases on your production SQL Server, instead install it to a seperate SQL instance. Means you'll be able to update Backup Exec during the day, rather then in the evening when your backups are probably supposed to be running.
See the following articles for their cop out reason for this stupid behaviour: