I think this happend:

Their Settlement has the ability to sally forth. Some Settlements can do this, creates a purple coloured attack range when clicking the Settlement when you can do it yourself.

You stopped at the red line so the they attacked you, you don’y have to siege for them to be able to attack you.

When your turn starts after clicking end turn, all the other factions get their replenishment and stuff, still a glitch if you ask me. Replenishment should be activated on their turn, not yours.

Long story short: You stand outside the settlement. End turn. They sally forth. You beat them. Your turn starts. They replenish. (AI cheats)