He did not appear to intentionally make a sentient car.
The extreme use the Harry and Ron put the car through seems to have pushed the magic on the car into new and unexpected ways. Mr.Weasleys own spell work appeared to be incomplete or shoddy since the invisibility button didn't work well from the start.
“Well, dear, I think you’ll find that he would be quite within the
law to do that, even if — er — he maybe would have done better
to, um, tell his wife the truth. . . . There’s a loophole in the law,
you’ll find. . . . As long as he wasn’t intending to fly the car, the fact
that the car could fly wouldn’t —”
“Not a word to Molly,” he whispered to Harry as he opened the
trunk and showed him how it had been magically expanded so that
the luggage fitted easily.
Then there was a popping noise and the car, Harry, and Ron
“Uh-oh,” said Ron, jabbing at the Invisibility Booster. “It’s
Ron may have accidentally added some magic to the car himself, while wacking it and breaking his wand on the dash.
“STOP! STOP!” he yelled, whacking the dashboard and the
windshield, but they were still plummeting, the ground flying up
toward them —
Magic-ing a car that simply kicks you out and drives off on its own is not a good idea for a ministry agent who could potentially loose his job having his car drive off on its own. So its most likely some unintended mix up between the multiple spells placed on the car.