The mind drifts like a mat in a open pool.
Do you ever space-out between quarters, halves, sets, scores?
Sometimes it's the mind's mechanism to rebalance.
But sometimes it's a lack of mental endurance.
The focus required in a disciplined yoga-asana practice can be mentally exhausting. And in that way, it is mentally strengthening.
The first step is realizing your mind's habits.
The second step is guiding it back to the task at hand...