Best guess is that tank fill pipe kinked, restricting flow rate into the tank so that fuel backs up and triggers shut-off on pump handle. Or, less likely, I think, possible that vent to fuel tank has mud wasp nest (depending on your part of the country) blocking air escape from tank as you fill it. Look under rig if fill pipe visible, inspect for kink or misalignment (run over anything big lately?). One last thought, does the unit have one of those old style filler 'guard' things that restrict the size / type of fuel nozzle you can put in it with the little 'gate' or 'door' that shuts, and if so, is it still in it's proper position right at the entrance of the fill pipe?