It's just the flight or fight response. Blood pressure and heart rate elevates for more energy, in addition to an increased blood flow for better movement. Pretty much everyone who does PvP gets it. The reason why we experience it to another level on perpheads compared to other games, is due to immersion.
The more "real" the game feels, the more it's going to trick your nervous system into going into the flight or fight response. Gunshots are insanely loud and genuinely scary sounding and you die in only a few shots. More obscure forms of immersion involve you investing your time and energy into that thing, therefore making it more real. For instance, you needing to craft and spend money on your guns makes dying much worse than dying in Call of Duty. If you're involved in org battles, there's also the loss of prestige against your rivals and loss of morale among your org when losing the raid.