As pathdependant said, this is a very common shady tactic in the ad business. You drive up your competitors costs with bots in order to drive them away from you

As pathdependant said, this is a very common shady tactic in the ad business. You drive up your competitors costs with bots in order to drive them away from your keywords or make them think their conversions on those keywords are awful and then abandon them. By far the most likely explanation in my opinion. Even if it's not the correct one, some of the traffic is almost certainly these kinds of bots.