This appears to be a verified bug with Danse, as listed on the Wiki:
If the player character has assigned Danse in Boston Airport and completed the quest Blind Betrayal, he will go back to the airport, sometimes spawning in the middle of the warehouse. Since the Brotherhood of Steel members will attack him on sight, the settlers might join the firefight as well, triggering a massive firefight. Being essential characters, the settlers will overpower most of the BoS members stationed in the airport, save for the essential NPCs like Madison Li, Ingram, Gavil, and Lucia. The only way of "fixing" this is letting Danse and settlers clean the place up and when the essential BoS NPCs are down, talk to Danse immediately and ask him to join. Now leave the airport and fast-travel to another location so one can safely re-assign him to another settlement.[verified]
Whether or not you had him assigned to Boston Airport doesn't really matter I would think, but it would make sense as to why he is suddenly there. The fix is to let the fight carry out until the essential Brotherhood members are down are Danse can be spoken to. Talk to Danse quickly, and have him follow you. Then fast-travel out of there and reassign Danse to another settlement.