The way Play of the Game works right now is that it's based on your score, which is then modified by being near an objective. As a result, you'll often see a Play of the Game being awarded to players who happen to get kill streaks close to a payload or capture point within a short period of time.
This is only an early iteration of the feature, though, and certainly not what we envision as its end state. We're currently working on ways to recognize players with Play of the Game for doing things that aren't just kill streaks-for example, if a Mercy resurrects her team and then goes on to win the match or a Zenyatta sustains his team through a difficult push with a well-timed Transcendence, etc.