Because the other players were well geared and boosting you in a rift level far beyond your own capabilities.
For a lot of players , their seasonal characters are now very well geared and can carry a non contributing character through very high level rifts, if you dont have the gear yourself even a skeleton will one-shot you and if youre low on Paragon levels a high GR will contain enough experience to boost by far more than just 100 or so.
Judging by the post, Id suggest sending a whisper thanking them for the boost as it seems you didnt realise that was what they were doing
This is the answer for you, all said there.
Not at all, just look the post I quoted above, this works for consoles aswell and does not need any cheats. If you look the other way around, this is also cheating, by boosting someone above what you can do.