Saw video from highly reputable source, a Disney CM who has many friends working at MK. She said this is what happened, not rumors.
The mass exodus after fireworks on ramp to monorail.
Two "individuals/groups" started an altercation.
One reached in to their pocket.
A guest who saw them started SCREAMING "g-n"!!!!
To be clear no "g-n" was ever seen.
People panicked and turned to go back down ramp.
Some "trampling" was happening.
Security locked down MK, logically so no more people exited, until resolved.
Monorails were stopped for about an hour (with guests on them).
Folks in MK turned and ran inward filling stores, screaming, heading back farther in park. No reason other than people started running, then others followed with no idea why.
Videos of strollers (personal ones) all abandoned on Main Street.
I've also seen post video of the Emporium and area, and folks just made a mess knocking merchandise on the floor. Lots of bad behavior by guests being reported. CM's took the brunt of it sadly.
People just can't behave ....