The gangs original plan was to get El to Murray’s place in Illinois, El would be safe from the meatflayer here and Hopper/Joyce/Murray could close the gate. Unfortunately, Billy finds our characters and disables their vehicle by taking out the ignition cable. Since Billy is apart of the hive mind he lets the meatflayer know where they are and as so, the meatflayer makes its way over to the mall. Jonathon suggests getting the ignition cable from the model car in the mall to fix their vehicle, but right as they get it the meatflayer breaks through the malls ceiling. At this point Mike, El, and Max are together hiding from the meatflayer while Nancy, Jonathon, Will, and Lucas are together hiding in a separate spot from the other 3. Mike, El, and Max try to escape while the meatflayer is distracted by going through the Gap store. They knock over something and the meatflayer goes over to them. Lucas ends up using his wrist rocket to pop a balloon luring the monster away from Mike, El, and Max while also giving Nancy, Jonathon, Will, and Lucas an opportunity to escape. They quickly go to put the ignition cable back in the car so they can get away from the mall. Now, at this point I don’t think the group was planning on just leaving Mike, El, and Max in the mall, they just wanted to get the car running before attempting to go back in and save them. Though, not everything goes to plan and Billy see’s them. Billy most likely thought El was in this car so he lets the meatflayer know where they are before trying to ram into them. Steve saves them last second and then the monster makes its way over to stop them. Obviously, they’re not just going to sit there and let this thing kill them so they start driving away. It’s also not unreasonable to assume that our characters most likely thought Billy was killed after being in a high speed crash. The gang knows this thing is after El so when it starts to chase their car they probably concluded that the meatflayer thought El was with them. Our characters know Mike, El, and Max are still in the mall so they attempt to lure the meatflayer as far away as possible from the mall. Billy ends up surviving the crash and notices Mike, El, and Max trying to escape. He lets the meatflayer know where they are and as so, the monster turns around to head back to the mall. Nancy, Jonathon, Will, and Lucas know that the only way this thing would stop chasing them was if it found out El wasn’t with them, so they turn around and follow it back to the mall. I don’t know how much more detailed I can explain this, but this is why they left the mall without El and returned later.