They don’t.

They’re trolling, no point feeding the troll.

Shocking.

GoT’s final half and definitely last season pissed me off to no end at the time, but in retrospect I’m glad I don’t have to care about this anymore.

In canon (hue hue) nah, “God did it” which explains anything.

Vagal tone, is the technical answer. A nerve (vagus) runs through your body and, among other things, slows the heart. When straining to “void”, you cause that nerve to activate more strongly. It’s also related to the Valsalva maneuver.