As far as I can tell this is caused by a respawn bug. After the first time you kill an enemy.. every time the respawned version of that enemy gets killed when you are in the area you might get xp from it. I was doing a full exploration run of the game so it took me a long time to notice.. but now, simply running through any area with heavy fighting between enemy factions, I'll start getting xp from kills I'm not involved in. I have no idea if it's caused by reaching a certain amount of enemies killed.. (I've killed over 2000 human enemies alone.) or if it occurs with every enemy respawn.. I'm thinking it has to do with the amount. I mean, I ran back and forth from places quite a lot in the beginning of the game. Plenty of fighting going on between factions and I did not get bonus xp from enemy kills. It was only after I had reached downtown and had explored half the map that it started happening to me.
In all reality though, you don't actually gain much xp from kills. It's definitely not as big of an xp boost as simply building and crafting. To make survival mode truly survival they should remove crafting and building as a way of earning xp. Grinding by killing is a lot harder than grinding from building.