In general, the warning indicates that the server's engines are being kept too busy to process the deferred queue. The issue can resolve itself if an engine finds cycles to work on the deferred queue.
Were there any other errors in the errorlog around this time, particularly error 814 or timeslice errors? If you are seeing 814, the issue is probably due to a known bug, CR 750807, which is fixed in 15.7 SP120.
Example:
00:0002:00000:00102:2013/11/06 22:17:14.61 server Error: 814, Severity: 20, State: 7
00:0002:00000:00102:2013/11/06 22:17:14.61 server Keep count of buffer '0x2a6682628' in cache 'default data cache' holding logical page '725505' in database 'foo' has become negative.