Every second, there is a 1 in 10,000 chance that a skeleton will suddenly sprint across your screen, accompanied by a sound.
The event is completely random, extremely rare, and has no effect on gameplay. You may forget that the mod is installed entirely until the skeleton finally appears at the worst possible moment.
This is a simple client-side joke mod created purely for unexpected moments and harmless jumpscares.
Why does the skeleton run across your screen?
Nobody knows.