You ask this question while he's in the middle of a streak where he's lighting it up like he used to. He's on fire with 9 goals in the last 7 games.
As for the cause of his drop off... coaching. When he was tearing it up and potting 50 a year, the Capitals were all offense under Boudreau. Fire wagon hockey they call it. Ovechkin was super productive and the team was fun to watch, but they went nowhere in the playoffs in consecutive years. A waste.
Last season, the Capitals switched to more defensive style of play (in hopes of it paying dividends in the playoffs). Their penalty kill and goals allowed were both in the top 5 of the league. Their defense was very good, but it came at the expense of the explosive offense that they were famous for. When the Caps won the presidents trophy in 2010, their goals per game was an incredible 3.82. That next year when they switched to a defensive mindset, it dropped to 2.67. This year its at 2.67 again.
Ovechkin naturally didn't enjoy the switch in philosophy and essentially got Boudreau fired this year through his lack of production and effort. Hunter was brought in and as far as anyone can tell, he hasn't done much. There was no great change in the team's play or style and they just kinda stayed in the rut that they were in leading up to Boudreau's firing. Having Backstrom out since January hasn't helped anything either.
Now with the playoffs literally on the line, Ovechkin has started to look like vintage Ovechkin. Not sure what the change is, but it's been nice to see (save for when he embarassed Keith last week... that wasn't nice).
