Security through obscurity is one reason. A lot of script kiddies will blast away at services on well known ports. Simply changing the port off the default cuts down on a lot of script based attacks, inhibits generic port scans, and makes it easier to differentiate a deliberate, human attacker from a script.

In this specific case, I don't know. Perhaps rotating through ephemeral ports is done to mess with simple traffic filtering rules on firewalls?