For reasons only they could tell you there are a lot of hackers screwing with people. One big hack now is the NG+ hack, where the hacker literally sends other players to the start of a new NG cycle. They also will hack you into an endless death loop. It also was a hack to inject items into another players inventory. That would get the guy banned by the games anti-cheat software. That doesn’t happen anymore.
Look at the DS3 forum on Steam for all the crap currently going on with DS3 online.
So what do you do about it? You should always have your own save. There is also software called Blue Sentinel that you can get that will detect cheats and will automatically make backups. It is safe as the game does not consider it a bannable mod.
The DS games apparently are easily modable, something which can make the game more interesting but also more hacksble.