Most hacked bitcoiner cause of using malware/phishing wallet/software/website, some of them while using electrum and got phished by installing from malicious server the prompts as a notification. While some of them while using exchanges that were hacked.
The lesson is always verify the source/signature of every wallet you want to install on your device. And don't ever put your bitcoin on exchanges for a very long time or as an alternative for wallet.
This guy gets it, most of it happens on windows computers, I know of a guy who lost his on a slightly different way, he had a keylogger on his pc, poor dude was just importing his btc wallet, a few days after he fixes his pc's "slow downs" it was already too late, he lost 3 btc. This was about a month ago, so yeah, BE CAREFUL!