Usually switching means that one lost a signal and so it logged back in and found a different connection.
I do not believe Networkmanger will disconnect a working connection and switch to another.
My laptop would at random lose wifi and sometimes reconnect and I "fixed" that by getting a new $20 intel m2 card to replace the one already in the laptop and that works much better. My old card was not an intel.
What kind of wifi card do you have? And do you have the 2 wifi networks named differently? even if you had a prefer one, if that wifi was down at that instant it would pick the other one.