As Grant Fritchey said single update statement can not cause deadlock to itself. So run sp_who2 which will give you what processes are running at the time. Look for blyby column. Look which SP ID this update statement has, and see if there is any non zero value in bklby Column. If you see something there, see if that SP ID is blocked by other ans so on.
You can get the text of SP ID by runinning dbcc inputbuffer (spid).
You can run profiler trace as sqlnaive sugegsted and also run DMV's as Grant said. But SP_who2 is the first palce where I would start.