When they taught in the 50s the story of where the moon came from, they had diagrams showing the moon material being ripped from one side of the earth's crust presumably where the Pacific Basin is today. Something similar could have happened to Mars, but there is of course nothing left of this material except the two small pieces Phobos and Deimos, if they came from that event and aren't just some kind of captured asteroids. There is no reason to expect that a captured asteroid would settle into a circular orbit except that is what happens apparently around Jupiter and Saturn. What could circularize such orbits when there are only a couple of small moons?
The faster something travels the more speed it loses when it collects space dust, meteors, etc., perhaps. If there was a giant disentegration of a large planet, then all the extra dust and debris in the inner solar system would serve as a drag on any object in an elliptical orbit because objects in an elliptical orbits would have portions of their orbits where they are going faster than the dust and objects their colliding with before gravitational attraction sped up the dust and debris right before impact. It would be more of a drag when the object is going fast and less of a drag or perhaps even a push when the object is going slow in it's various positions of it's elliptical orbit. Over time, that would circularize the orbit, I would think, especially on a relatively small planet like Mars, or on a small moon like Phobos. As the inner solar system has been cleaned up over time, this drag is less apparent now but it could've been a lot greater right after the event. Maybe Mars hasn't collected enough debris and dust to circularize it's orbit, I don't know. From what I understand Mars orbit isn't all that stable. It could mean it hasn't been where it's at for very long.