It happens when you don't have sufficient free space in your disk pool for a
client to send its backup data.

The client doesn't just wait - TSM automatically tries to switch the client
to direct-to-tape operation rather than failing the backup.  THEN the client
will wait until a mount point (drive) becomes free.  (Unless you specify
MAXNUMMP=0 in the definition of the node when it is registered.)

That appears to be what is happening, since you have a migration for disk
pool SERVER in progress, and it's still 80% full.

The pool doesn't have to be totally full to trigger this condition, either -
just too full for the largest thing the client wants to send.

So if there is still some free space in the pool, you may have some clients
still backing up successfully to disk, while others grab a tape.

However, once a client is queued to wait for a tape (mount point), even if
migration does clear some space in the disk pool, the client will not get
switched BACK to the disk pool, it will still finish its backup direct to
tape.

There is nothing really wrong with this; the data is getting where it is
supposed to go, and your backups are working and not failing due to the
disk pool filling up (that's what TSM is supposed to do for you, yes?) So
you can just ignore it!

Or, here are some things you can do if you need to PREVENT clients using the
tape drives:

Add more space to the disk pool, as you are planning to do

Force a migration of the disk pool down to 0 before most of these backups
start, to make sure you have the max available amount of free space.

Turn on compression on the client, so less data comes into the disk pool
(Now this has it's own potential drawbacks - check the list archives at
www.adsm.org for all the pros & cons of using client compression.  But not
having enough disk pool space is one of the reasons TO use client
compression.)

Reschedule your clients a bit so that your data arrival is spread out
more.

Set MIGPROCESS=2 on your disk pool, so that when a migration IS triggered,
you get two output tapes mounted and clear the pool out twice as fast.

Those are just some of the things you can do.  But again, you don't have to,
your backups are getting done as is!