If the filling of a fruit pie bubbles during baking, what is the likely issue?

When a fruit pie filling bubbles excessively during baking, it is often an indication that the filling was not adequately thickened prior to baking. A properly thickened filling will have the right consistency to hold together and prevent excessive bubbling. If the filling is too liquid, it may bubble over as it heats up and reaches its boiling point, leading to overflow and mess.

While a hot oven can contribute to rapid boiling and bubbling, the root concern is the consistency of the filling itself. If it lacks proper thickening agents like cornstarch or flour, it cannot form a cohesive structure, resulting in bubbling during baking. Thus, the most likely explanation for the bubbling is related to the thickening process of the filling.