When you shift into a gear, it is under the assumption that you are going to drive away - and disconnect from Wifi. Rather than wait to lose the signal, and then attempt to access the cell network, it immediately goes off Wifi. If you want to connect while driving, you have to manually connect each time. I seem to remember this was implemented somewhere in the 7.1 timeframe.
This approach makes complete sense. If you are streaming when you first select a gear, the transition from WiFi to cell is fairly seamless. Otherwise, you drive away, lose the Wifi signal, streaming dies, then the switchover to celluar, then your music resumes.