Case fans are rotating, connected via Displayport 2.0
Case fans are rotating, connected via Displayport 2.0
In short, the display port is likely supplying power from your monitor through the graphics card. The fans keep spinning as long as the DP remains connected, but stop when the monitor loses power. This behavior suggests the monitor is drawing power via the DP cable. It could be a minor issue, but it might cause boot loops or instability. Cleaning dust from the port and cables should help.
This seems unusual. It might be a short on the motherboard causing the fans to start up, but you noted they tend to stay on after shutdown, which could point to a bad capacitor. I'm not fully familiar with electrical parts, so I shouldn't take this at face value. Could also be related to the Display Port Cable being connected to the monitor or just the cord?