Yes, OK, it's his preferred lane. So what?
Let's work through it logically. There are four lanes only lane 2 is occupied.
So if he chooses lane 2, it means one of two things. Either he wants to swim in lane 2 because it is his preferred lane, or he wants to swim in lane 2 because it is the lane the OP (a lone female stranger) is swimming in.
When he gets in, the OP has no way of knowing which of the above two reasons is his reason for choosing lane 2. If it is because he simply prefers lane 2, this is annoying because he has chosen to get in her way when the rest of the pool is empty. If it is because she is there, she is potentially in danger from this random man. Either way, it's not good for her.
If it turns out that he does not represent any danger to the OP but simply prefers lane 2 (and once again, she has no way of knowing this), there are three possible outcomes.
- She moves over and he gets what he wants.
- She doesn't move over and he gets the message and moves over.
- Neither of them moves over and they continue swimming in the same lane.
The fact that he chose lane 2 and subsequently moved over when she did not suggests that he chose lane 2 because it was his preferred lane, but he also wanted a lane to himself. His first preference was to swim in his preferred lane and for the OP to move, (i.e. his intention was to try to make her move out of the lane he wanted to swim in), but his second preference was to swim in a different lane and have it all to himself. When the OP did not move, he switched to his second preference.
Why does this matter?
Because his actions show that he consciously chose to make a woman (and, as a reminder, the OP was a lone female stranger) feel uncomfortable so that he could get what he wanted.
That's why it's arsehole behaviour.