Why Does An Enemy Planet Get Reinforcements After Its Warp Gate Is Gone?
Q: I went into an enemy planet and destroyed the warp gate in order to protect my adjacent planets. But later, when I went back, I saw that there were even more enemy ships guarding that planet compared to when I made my raid. How is the AI getting those reinforcements in there?
A: Command Stations and some other structures (Special Forces Guard Posts, AI Troop Accelerators, etc) allow for the AI to reinforce a planet even after the Warp Gate is gone. Basically, any AI ships or structures that have the Warp Gate (Full) or Warp Gate (Reinforce) abilities.
So as long as the AI controls a planet, it can reinforce it if it so chooses (the AI gets a limited number of reinforcements per period of time, so it won't always reinforce all its planets adjacent to your planets). When a reinforcement occurs, the AI adds ships at the command station (if it still exists), as well as at all the guard posts.
See also Basic AI Reinforcements