We've all seen it... you're out walking or riding with the kids on a quiet trail on a Sunday morning when out of nowhere you hear someone scream "ON YOUR LEFT" .005 seconds before they blow past your left shoulder at 25mph with inches to spare - often while there is oncoming traffic.
These people are not announcing a pass or alerting you to their presence, they're telling you to get out of their way. They're usually in the middle of a workout and riding way too fast. They also probably think MUPs are a good place to do threshold intervals and don't understand why so many slower cyclists and walkers are out there clogging up the trail.
It's not really a mystery why some pedestrians and slower cyclists get a little prickly about the passing situation on MUPs.