Why does my apple crumble develop a bitter aftertaste?
It may be that the specific flavour has a compounding effect, in the same way that spciy heat (capsaicin) does. The first bite is great, the second one is more sour, and by the second portion it is the only thing you can taste.

It happens because of the way the mouth's flavour sensors react to certain flavour compounds, but I don't know enough to explain how it works exactly.