Patches can (and usually do) prompt for restart if they cannot themselves access/replace a file in use with a service, or restart the service itself. This is most common when the patch relates to a core part of the o/s.
Patches are monolithic unless rolled up into a service pack, and hence you could even get to the point of 1 patch = 1 restart.
There are scenarios whereby the patch delivery gets screwed though, and the same patch will request to be installed again and again, along with the associated reboot request. This is where you rebuild the SoftwareDistribution files as per the thread post I wrote a couple of weeks ago.
M.