The usual scenario is economy is oversold so they needed to bump someone up to business to make room for all the economy passengers. If they didn't want to break up a family and you were in a seat that would otherwise allow them to sit together then it's easier to move a single person. They also look at things like fare class and check in time, as well as status. Sometimes it's just random and you were an easy person to move.
Be grateful when it happens, and never expect it to happen. In 20 years of flying I have only had it happen once on AC and once from economy to premium economy on BA. Both times I was a single traveller.