TSM Support says this matched the symptoms of the APAR described at  
http://www-01.ibm.com/support/docview.wss?uid=swg1IC62990
In short, there is no known cause and no solution.

Here is an extract from the APAR:
This hang condition has been seen on the zLinux platform, but could
effect other Linux or Unix platforms, and other versions
of the Tivoli Storage Manager. The problem is completely random and is
not easily reproduced.
Initial Impact: Low - the upgrade is successful.
I don't agree with the 'Impact' assessment. Yes, the upgrade is
successful, but who knows?  When the hang occurs, one doesn't know
whether UPGRADEDB is doing something needed, or not. In the meantime a
Production system is out of service.
Best wishes,
Keith Arbogast