You asked: “Every citta has an Object associated with it. when we meditate on breath then the object of attention is breath.. and after some time suddenly a past memory image pops up.. can one explain in terms of Abhidhamma mind-moment model?”
Abhidhamma easily explains it. It is just that the author of that paper does not understand Abhidhamma.
The basic idea is explained in: “Change of Mindset Due to an Ārammaṇa”
There are six types of “thought objects” coming through the eyes, ears, nose, tongue, body,and mind.
– While one is engaged in breath meditation (which is NOT a Buddhist meditation, by the way) only the mind is active. Actually, the body is active too, since one is “monitoring the breath via touch sensation.” However, since you are talking about random thoughts that come to the mind, those come in through mind: “manañca paṭicca dhamme ca uppajjāti manoviññāṇaṃ.”
Please see, “Chachakka Sutta – Six Types of Vipāka Viññāna”
I am not sure how much background you have on Abhidhamma. You can take a look at the Abhidhamma section: