Since it didn't happen on hundreds of other carriers, it hardly is an iOS bug.
What is more likely? There is a bug in iOS or a carrier is able to send some command wirelessly (by accident or on purpose) to cause an iPhone or any smartphone to switch from WiFi to mobile data connection? From iOS' view, why would it connect to a mobile data connection if a WiFi connection is available (assuming iOS is bug free).
If you think a carrier is able to do that, why does iOS acknowledge that command? Why not go full crazy and believe that Apple made some back door for the NSA and AT&T found out about this backdoor?!