I'll be honest with you. I have no idea why he woke up. Sometimes you don't know why your child does certain things. The thing I did know was that this waking was unexpected and I always check on the unexpected. If your child knows how to fall asleep and stay asleep on his own and doesn't need nightly feeds, any other waking will be probably** be unusual and unexpected.
With unexpected wakings it is a good idea to check on your child. Try to figure out what is wrong and let that guide your next move. If your child is in the process of sleep training that involves you being away from his presence while he learns to put himself to sleep and stay asleep (with or without crying) then figuring out what is usual and unusual will be a bit more tricky.
A video monitor can help you figure out what is going on (and it can be addicting to look at!) Sometimes some trial and error will be your best bet (always check on baby if you are unsure). And of course there are many articles on this blog in the blog index to help you out too. Whatever you do, remember to listen to your motherly intuition (or whatever you want to call it). It will help you know when to check on your baby and what to do when you check on your baby.
Maybe you will figure out the issue and decide, like with a child practicing crawling, that the best thing to do may be to leave him alone. Maybe, like with a sick child, you will offer some comfort but try not to give more than is needed so that you don't create new habits that need to be broken later on. Maybe you will notice that checking in on your child is making sleep progressively worse and decide that it is time to decrease your attention during sleep times. What you choose to do to comfort your child will vary depending on your child's specific needs at that time. Over time you will learn what works best in different situations.
On a side note, some of you may have noticed that I picked up baby Jacob (ok, he's not really a baby anymore, but I'm pretty sure he'll be my baby Jacob forever). This is a big no no to some people. I disagree, especially if your child has some solid sleeping habits. I have found during sleep training that some kids do better when they are picked up while it is really detrimental to others. Also, once a child sleeps well on his own, most kids will continue to sleep great even when you hold them to sleep every once in a while or do something else you normally wouldn't do in the middle of sleep training. If this isn't your child, well, then be a bit more careful with the extra attention you give during sleep times.