You say "constant distraction is a defense mechanism" ... against what? Just resting? Quiet? Peace? Stillness?
Yes, that's pretty much it.
Actually, mental activity is always very busy, but because we are in the midst of it, we don't usually realize it.
When you do Shamatha (mind calming meditation, watching the breath, etc.) it feels like your mind is fighting back, getting busier. Actually, you are merely noticing how busy your mind is all the time.
The busy-ness of the mind has to do with awareness and objects of awareness. That means that you have all of this sensory apparatus all set up ready to go, in constant use, and it places "you" in the context of everything else. For example, suppose you have a spoon, you are holding a spoon in your hand. Because there is a spoon, and you are aware of it, that establishes a sense of "you" in relation to that spoon. Now, if you extend the example of the spoon to everything else in your world that you have contact with, you can see where a very solid experience of a self constantly happens. There is a constant subject-object activity going on.
When you do mind-calming meditation, you regard all of these 'me' reminders merely as thoughts, and return to watching your breath, or counting or whatever your particular meditation practice uses as a focus.
So, if you are only watching your breath, for example, the 'monkey mind' is what is grabbing out in desperation, this way and that way, trying to find all the things that help solidify your experience as "self"... all the things you need to do, memories of childhood, misc. worries, funny thoughts and so forth. It's like a child that doesn't want to take a nap, and is running around trying to find another toy to play with, trying to do anything except lie down and take a nap.
What is happening is that meditation threatens the "self' experience, because it ignores all of the props that the 'self' uses as, well, sort of as mirrors. That's why a lot of people (think they) can't do it, or don't want to do it, they get all fidgety and restless and uncomfortable and bored.