Numerous steam accounts, primarily of twitch streamers, got hacked recently. Through a mistake in the login feature, hackers accessed the accounts and were able to change the password without triggering an e-mail to the account owner.
A flawed steam client version is to blame for it, that it only needed a username to set back the password. The unknown hackers just had to press on a “next” button without entering any safety code to get into the user settings.
Also affected was Youtuber Elm Hoe who released a Video which shows, how the accounts were hacked: https://www.youtube.com/watch?v=QPl_BJoBaVA .
Meanwhile steam support has solved the problem and released an update which fixed the security vulnerability. Furthermore they blocked the trading system on those accounts so that the collected items can´t get trade away.
At present steam has not given an official position and all the information come from https://www.reddit.com/r/Steam/comments/3elt4w/several_twitch_streamers_just_got_hijacked_and/