@Jeremy564 As a host, Airbnb sends an amount like that to hosts' accounts when they first set up their listings to make sure the payment path works, but I don't know why they would charge an amount like that to a guest.
It's probably a good idea anyway to remove your card info from the site if you aren't in the process of booking or staying at an Airbnb. I had a guest who told me she always does that, since her account had gotten hacked a couple years previous.
You should ask Airbnb about this, but currently their customer service is incredibly slow. But you could send them a message through their Twitter account. You might also want to change your account password, just to be on the safe side.