Feeling the need to justify their words/actions all the time, and explaining a little bit too much, instead of just relaxing, and giving the people around them enough credit to be able to surmise why.

Eg:

Normal: I'm just going to the bathroom [puts bag on chair]

A tad awkward: I'm just going to the bathroom, i've been drinking loads of water due to my new diet, i'll leave my bag here, just telling you so it doesn't get stolen, like this other time last year when I was at a meeting, i'll lean it against the back of the chair so it doesn't fall off.

Not being able to pick up when someone else is completely disinterested in what you are talking about