So i was quietly minding my own bussines as the vaulters, Horatio declared war on me (no biggie, could handle that), and then 2 turns later a pirate fleet spawnd at EVERY system I owned, and they all invaded where I could only use stampede as defense

So I assume I was hacked (as happines was estatic), but what wildly unbalanced outcome does this? I protected my home system, but it never seems to do anything.

This, combined with getting attacked, is an instant game over... it's even worse than Juggernauts and planet destroyers....

Yeah except you can actually protect your systems with improvements..

Yeah, pretty sure your home system was hacked and action: "Support Revolutionaries"  was chosen by your opponent, what leads to

a) pirate fleets spawn in orbit of your systems (taken from your systems population) and 

b) stampede as defense for each of your systems is activated

Guess you played multiplayer game and your enemy did know what he/she was doing. Same time you didn't seem to care for to build system defenses/raise manpower in your systems at all - not to talk about thoughtful placing of defensive hacking programs. Else a one-time "Support Revolutionaries" action shouldn't cause an instant game over. Annoying, for sure, but not impossible to prevent from. Perhaps you should turn off Penumbra DLC and/or choose to not play against more experienced gamers.

Perhaps this guide is of use for you, to prevent from getting surprised again: Hacking and stuff by koxsos

Place defence programs not on home system node, but around it.

Sometimes, feasibly protecting your home system from hacks just isn't possible. When this is the case, there's nothing you can really do to prevent your enemy from doing whatever they want to you.