Yes , the Captain and First officer will know why. This is a bit like saying "I saw a car on the hard shoulder of the M6. Anybody know why ?" Aircraft go around for any number of reasons. If there is a delay in configuring the aircraft because of a flap caution or some such minor thing, then the crew would need more time to resolve the fault. If the aircraft was not profiled for any number of reasons it might be necessary to go around. If the runway needed to be inspected or wasn't anticipated to be clear in time it might result in a go around. There are a whole host of factors that have to be satisfied for an aircraft to be able to complete a landing from an approach. In 99.9% of cases they are, but on that arbitary figure it would still leave 1 in every 1000 approaches resulting in a go around say a couple every week big deal !
A go around as is frequently pointed out on these forums, may be something of a spotters dream, but it is a normal manouever albeit one that is done relatively infrequently.