DIAGNOSIS The rubber dust port collar loses its grip, especially if it's been stretched by a tight hose or if sawdust and resin have made it slick. It's a friction fit, and that friction wears out.
FIX The best permanent fix is to add a mechanical clamp. Get a small hose clamp from the hardware store that fits over your vacuum hose. Connect the hose to the saw's port, then tighten the hose clamp over the rubber collar of the port. This creates a positive lock that won't pop off from vibration or hose drag. Clean the rubber port with soap and water or rubbing alcohol first to remove any slick residue.
IF THAT DIDN'T WORK If the rubber port itself is torn or permanently deformed, you can order a replacement part from Milwaukee (often called a dust chute or adapter). It's a user-replaceable part.
HEADS UP Don't try to rig it with duct tape over the long term; it'll leave a gummy mess and doesn't solve the vibration issue. Also, using a shop vac with high suction without a good connection can actually pull the port off, so a clamp is the right solution.