[_98fj]

Not meaning to get too personal here, so feel free to ignore this, but it still could be that you're zoning out to avoid some negative emotions.

Could it be that you're dreading something that is "implied" by the lecture in a way? Maybe the practise that you feel obliged to do? Maybe the impact the lecture could have on your work or schedule? Maybe just the end of the lecture and back to whatever comes after?