SQL Server CPU spikes - possibly non-query related - suggested indicators
The first thing I would suggest you check is your "max degree of parallelism" via sp_configure.  By default this is set to 0, so a parallel query will consume all CPU on the box (up to 64 threads) until it finishes if this setting is unchanged.  I've seen this occur on more systems than I care to admit over my career, and it tends to be even more frustrating on the larger servers (with a higher number of CPUs).  If this is set to the default of 0, the recommendation from MS on what this should be can be found here: http://support.microsoft.com/kb/2806535

If it's a smaller server with very few CPUs (e.g. 1 or 2) it could be something as trivial as auto-update statistics kicking in, though that has been reportedly rare... but rare still happens.  You'd have to check if that is the culprit via a profiler trace to be certain though.