First of all, for each combat field, the list of possible enemies is quite limited - defined by local faction, law level etc. It is indeed possible that for certain locations the list effectively consists of just 1 or 2 entries. In your example it could be 1) Ossiri Raider and 2) Bruwar Turret (and maybe something else, the "5th ship" you describe). Again, it is possible that the "biodiversity" is unintentionally too low in some fields. I'm sure more and more enemy types will be implemented over time, increasing the balance.
At the same time, the spawn chance is equal for all enemies on this list. Meaning, if you keep killing one specific enemy type (e.g. for a mission), you're effectively increasing the population of "everything else" in the field. In you example, you kill all Turrets as soon as they spawn, which results in more and more Ossiri Raiders over time. Actually, an interesting side effect of combat missions Aside from the possibility of group creation^^ you have to start killing other enemies in order to allow mission targets to spawn. Though it was stated many time already, that exactly this is intended: combat missions provide an additional motivation/reward (especially faction xp) for combat you would do anyway - similar to hydra missions. You're not meant to be able to kill mission targets in quick succession.
As Ossiri Raiders belong to the same faction, they act as a group. Meaning, aside from the standard random chance of someone attacking you on their own, the group uses its combined combat strength vs. your ship as decision basis.
Now while one Ossiri Raider is a rather mediocre enemy (~ medium tier1 player ship) and probably wouldn't attack (often), 3 or even more are a very dangerous pack, especially considering their high DPS - they can inflict significant damage to any low-mid level player and they know it As group attack is way more promising than a personal attack, it is very likely to have higher priority - resulting in a massive attack (almost) each time.
Hehe and imagine how surprised other players will be if they enter that "trap" Well, you would either need a group to clear that field - or you can wait long enough, they'll eventually be replaced (though we're speaking about many days here).
Now, the repeated word "immediately" was a little bit confusing. The general drag-in cooldown should still apply. That is, if you were attacked, have just retreated and re-enter a combat field immediately, the chance for an immediate attack is exactly 0%. It then increases over time - relatively fast in combat. Though in few minutes it reaches a significant value - and if enemies are much stronger than you (group^^), they are very likely to attack. That is, "immediately" cannot be literally upon re-entering, but it's probably a matter of few minutes for the above constellation.