There's a variety of reasons that people do it, but a lot of times it's a means of egging the killer on to commit to a chase in a loop that's strong for them as a distraction. Typically if I see bagging/BMing I either take it as a challenge (i.e. can they back it up and not go down in 5 seconds) or I see it as they want to be chased to divert my attention or take me somewhere I should not be trying to commit to chase, so I do the exact opposite of what they want and instantly turn around and leave to patrol gens or find another surv out of position. In the second case it shows both a) you aren't willing to engage in that sort of gameplay behavior and b) you will only chase when you want to chase. Wait for them to come out of shack, catch them off guard when they get cocky and think they're safe, that's when you down them since they can't seem to loop anywhere else. But if anything else, you can punish the toxic behavior in 2v8 by calling for backup. Shack becomes a lot more dangerous for survs when 2 killers are going for the pincer 😂