gas pump keeps shutting off fueling, why?

On some vehicles, on the filler tube from the neck to the tank, there is an air line which allows air out of the tank while the gas goes in. If it is blocked or broken the air not filtering out will cause the pump to shut off from back pressure. That, or an "ex" or someone else shoved something down the filler tube.

(really liked previous answer) but to add, some pump stations seem to pump faster than others, or so it seems, and that might overwhelm the system setup as the previous guy mentioned. I have to wonder if this is happening at the same station or at any station.

after two fill-ups and 500+ miles, it now fills properly. may have cured itself?

Sounds like a clog/plug in the EVAP system to me. If it reoccurs chech the lines on the canister for debris.

Since you say it fixed itself... Overfilling clogs charcoal canister, works itself out eventually. Don't top off.