With MN winter comes ice, and ice can play havoc with hydraulics. I suspect that you have some ice crystals being stirred around by the tractor's hydraulic system. If there is a lot of water=ice, then it will go to the bottom of the sump. But with our current "warm" temps we can get ice crystals within the oil. The ice crystals can mess with the pressure relief and exhaust valves and may make the pump act eratically.
PS. Don't get your body parts in such a place that a falling bucket or load can trap or injure you. If you want to leave the loaded bucket partially up, then block it up. Stay safe!!
Paul in MN