If the wireless network you are connected to has a poor quality, your device will automatically switch to a better Wi-Fi network, if the new network is within range and you have previously connected to it.

If you want to cancel the new connection, open the Settings on your device, and go to Wi-Fi (or Wireless & networks > Wi-Fi for devices running EMUI 9.X or below).

In the list of available networks tap on the name of the new network and select Forget.