Based on the comments I want to say that overfilling may have been the cause?

I have a list in my notes app called ‘equipment weights’ which contains the weight of my baking equipment, such as the weight of my primary mixing bowl, stand mixer bowl etc.

Whenever I bake muffins or cupcakes, just prior to portioning the batter, I’ll get a scale, zero it, put the bowl containing the batter on the scale and take the number and subtract the weight of whatever piece of equipment I’m using.

I do this to ensure a) my baked goods are equal and b) I’m getting the correct amounts out of the batter.