I agree - I have a hard time imagining that 15 minute backups of a tlog are unusual.  However there does seem to be a direct correlation between the increase in frequency and the increase in application 'hangs'.  Even more to the point, the times of the application hangs often correspond with the kickoff of a backup.  I will look at the possibility that someone has checked the option to run a DBCC package.  But this is just strange.