I thought that the person with the highest elo gets to be the team captain.

I was second pick in a game where the team captain was 50+ points lower than I was after I checked the elos at the end summary screen. He wasn't even the person with the second highest elo.

It's not like I just started playing ranked where my elo junps high with wins.

So what determines who gets to ban?

Something to do with a person duo queuing, I think.

The captain of the duo queue gets first ban because it's assumed that he or she will have more coordination with his or her partner thus leading to more effective bans. Sadly this isnt always the case, but generally people who are duo queuing are doing something right.