What causes macarons to sticke to parchment paper?

If your macarons are not cooked all the way they will stick to your Parchment, give them another 3-5 minutes in the oven and let them cool down all the way before you try and peel them off of the parchment paper.

They are probably not cooked (dried) enough. I would leave them in the oven longer, until they easily come off the paper. If the tops are dry and you want to prevent them from cracking, cover the macarons with tin foil and put them back in the oven, lowering the temperature by 100 degrees. If you are working in a really humid area, try to remove them from the tray as soon as they cool because otherwise the humidity could make them soft enough to stick to the parchment paper.

To get them off the parchment paper just dab a slight bit of water on the back side of the paper and the cookies will come right off

I would start by baking them on silpats, and try baking them a minute longer. Next, run a small offset spatula gently underneath the macaron to remove it.

Maybe let them rest a bit longer before you try to remove them.
You could also spray paper lightly with a flour/oil 'bakers' spray.

But I would strongly suggest getting a re-useable sheet of "Silplat", a silicon flexable baking sheet; with excellent non stick properties.
Give a rinse and dry and roll it up and put in a empty paper towel tube for storage.