This is likely a consequence of the recent gravitational encounter. (sciencedaily.com)
On the other hand, if 30 percent or more of the mass of the cloud core goes into making stars, then the formed stars will remain bound to one another, and the ejection of stars by random gravitational encounters between cluster members will take many crossing times. (britannica.com)
One possible reason for the fresh surfaces, a team reports tomorrow in Nature, is that those asteroids had their surfaces disturbed by past close gravitational encounters with Earth. (sciencemag.org)