What is the cause of Muṣitasmṛtitā? I have read the causes of anger, lust, etc. but I haven't yet encountered the cause of Muṣitasmṛtitā. I am sure, most of us have had issues with this ailment. 

What is the root cause of it?
If you read my answer to question about attention, you will see how I explain the process of attending, which is exactly what you're asking about. 

From this explanation it becomes clear and almost obvious, that the cause of losing attention is that some other stimulus creates more active associations in you than the object you're trying to attend to. So those new associations crowd out perceptions and thoughts about the original object, and so you switch the focus.

The root of the problem is, you have more interest in other topics than in the object you're trying to focus on.

Maybe you have inner resistance/aversion to this object. Maybe you are lazy because you don't believe in success of your exercise. Maybe you are simply not familiar with the object of meditation and so you have nothing to think about it. Either way the object is not creating enough reaction in you to remain engaging.

One way to fix all this, instead of forcing your attention to stay there by force, is to do active investigation of the object you're attending to - to examine it, look at it from all sides, compare it, analyze it, research it, examine your attitude to the object, - and this way you will generate a lot of associations leading back to the same object, and so your attention will remain with it.