Help understanding what exactly happened to my BTC
Go through your browsing history if you really want to know.
No one here can tell you what happened due to lack of information, and it shouldn't be posted how it was done in the first place.
^^This^^
Also, on the reddit you've already been told that JavaScript on the webpage can't 'hack' you(99%). And guys there provided you with some possible scenarios.
Since for some reason you still need help in understanding what happened, I put it in simple words - you've(with your own hands) executed program which transferred your BTC to another address, assuming ofc it did actually happen.