Somehow when my outposts get attacked, I always have to immediatly fight the enemy. What do I have to do so that he has to siege (I have a guard contract)

after he defeats the province guards he has to lay siege on an outpost/fort or your stronghold - as long as you have at least one unit in the garrison on said outpost/fort

thats why it is good to always keep one inexpensive troop in these buildings

its not JUST the enemy.
Rarely I would capture a province with an outpost WITHOUT having to siege it first. because the AI didn't garrison troops in it.

Siege is ONLY required if there is a garrison inside the outpost.
On key hexes I build an outpost and put in a single militia (cheapest unit with lowest upkeep).
I do NOT put guards on it because the enemy will just kill them and gain XP. Instead the enemy sieges my outpost and I kill him with my hero (he CANNOT escape if he is attacked while besieging a garrison)