Is it a mental thing?

Why do people ult when someone else ults? Is it a panic type thing of "I don't knoe if it was my team or theirs" or is it like "they gonna steal my play"

I was reaper and ulted then i hear "NERF THIS!" "METEOR STRIKE" "HAMMER DOWN" "HEROES NEVER DIE" "YOUR POWERED UP GET IN THERE"

I just stood there in shock and it was even in the kill cam of me just on the payload and staring into space as everything unfolded