Any idea how this happened?

For 15 years this pear tree has born only green pears. This year, for the first time, loads of darkish red pears have joined them. Any clues as to variety and how this happened?

Is it a grafted tree? Can you single out a branch or branches that only red pears grow on? Any chance the red pears are growing from sucker branch that starts low down on the trunk?

Red pears all have their own branches, some low and some high. Red and green never appear on the same branch.

I suspect some sort of issue with the grafting rootstock growing into the tree, but the branches that are bearing the red fruit could also have been grafted onto the tree.

Look at the tree trunk and find the grafting union. There should be only one trunk and the grafting union should be a few inches above the soil. Also make sure that the soil is exactly at the level of the tree's root crown (where the tree starts flaring out at the bottom and the roots start). Do all the branches start after the grafting union?  if any start before the grafting union, they should be removed. If they all start after the grafting union, I'd look at the branches that are bearing the fruit to see if they were grafted on.