It probably was a bot. You have heard of ransomware - they operate by encrypting your profile and then demanding you pay bitcoins to clear it - these are malware infections, and you probably just suffered such a robbery... Bank robbers don't need guns anymore...
I run windows, but I am something of an IT expert, and I just basically don't run anything or visit sites that would have malware payloads. This is the real reason why you should be running adblockers, because, unfortunately, and a big fuck you to the advertising user-as-product business model who claim that there isn't a way to monetise digital content and services without advertising cough cough... Most such malwares arrive either in some crappy pirate software, or, more commonly now, injected into your web browser. It doesn't even have to escape the browser box to pull this off, though it has to hijack the pipe that connects each web page to the controlling application.
After such a horrible loss, I would suggest that you up your security and segregate either a computer, or an operating system, that is not used for general tasks or whatnot, just for money, and lock it down tight, strong password, and don't install anything more than the browser and cryptocoin wallets you need.
I am in the hole at the moment with poloniex to the tune of 1233 steem... I am kicking myself for using poloniex again, it's almost never been smooth or pleasant working with that site, and if I had just waited another 5 minutes for blocktrades.us to come back online, (and it did, directly afterwards), a little tiny bit more patience, I would not be in this situation...
I still want to go visit poloniex's offices and personally slap everyone who works there, and for good measure, I want the CEO's luxury car for my trouble.
Fucking criminals >_<
RE: $20,000 (7BTC) Stolen from Bittrex (captured live on video) Huge lesson learned