The fact that the ball was hit hard and got into Rocchio's glove very quickly makes it logical that he would think the right move would be to just step on second base. As the photo shows, Buxton wasn't even halfway to second when Rocchio had the ball in his glove. So I don't blame Rocchio at all for his decision to get the force.
His error, IMO, was that as he approached the base he had Buxton right in front of him and obviously realized it wasn't going to be the routine play he thought. At that point he should have simply dove for the bag and touched it with his glove rather than sliding in feet first.
If you are a runner leading off first base and the pitcher tries a pickoff, do you slide in feet first? No, you dive forward and reach with your hand. That's what Rocchio should have done. I was shocked to see him slide into second.
By the way, kudos to Clase for getting five outs that inning. But he needs to look the ball into his glove before looking for the bag. I'm sure he will next time. And I'll bet Josh Naylor never throws to third base again when he has a runner trapped between third and home. Buxton did a great job of deking him by faking a break for third to draw the throw and then reversing direction just as Josh was releasing the ball.