Some people are greyhounds, and will do anything to have a rabbit in front. So, they see you go by and the instincts cut in and they chase until they catch up, then pace.
The fact that it's happened often with no consequences tells you it's pretty benign.
Personally, I don't like being drafted because it imposes an added degree of care should I decide to slow or stop on whim. But I don't mind if they pace off me a few yards back.
So, ignore the greyhounds, unless you prefer to zig across a few feet and lay up, so they come alongside, then strike up a conversation.
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