Interestingly, I just started getting this message every morning. The one thing that changed is that I took the access point closest to the garage temporarily offline. My MS is now connecting to an AP further away, and the indicated signal strength went from 4-5 bars to one. So possible theories are: 1/ Even though the MS is indicating a weak signal, it isn't actually getting an IP address and therefore is not connected (layer 2 vs layer 3) and the lack of a connection forces the prompt/window. 2/ It could be programmed to prompt the user if the signal strength is weak and while scanning it sees stronger potential "better" candidate networks.
Edit: Just saw @sacrament055's note that he is prompted with a strong wifi connection, so that is a datapoint against my theory.
It could be a timing problem. The car wakes up, isn't yet connected to Wifi, prompts the user, and then subsequently the Wifi completes the connection. You would end up with the screen shot above - the user prompt with strong WiFi indicated.