Poor JavaScript or very old computer.

Often, it's a bug: blocking infinite loops

while(true) {}

// or, which happens very easily
eventA.on('fired' () => eventB.fire());
eventB.on('fired' () => eventA.fire());

// or
function foo() { /* bug in recursive code */ return foo(); }

It happens more and more because of all those useless Reacts, Angulars, overcomplicated JavaScript and other hype.

Those over 9 million young fancy JS and framework fanboyz and gals forgot/don't knwo that web so many years later is still only about retrieving simple information thought simple HTML text docs, web never was a desktop or gaming environment.

Basically browser freezes because JS does/loads a lot of resources at that time on bg. Firefox at least should drop a warning that "certain page runs slow, do you want to disable JS".