This report describes the collision of the French satellite Cerise with a piece of Ariane rocket debris (object no. 18208) in 1996: http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1997ESASP.393..589A&defaultprint=YES&filetype=.pdf
The approach angle between the two objects was 158.9 degrees (nearly head-on).
Near the end of section 4.4, the report states: "It is astonishing to note that the period and hence the semi-major axis of object no. 18208 increased during the presumed collision whereas the two objects were going in opposite directions. The delta-V in the trajectory of debris no. 18208 is in the direction of its velocity and not in the opposite direction."
Has the increase in delta-V of the Ariane debris ever been satisfactorily explained? If not, what are some possible explanations for how a near head-on collision of two orbital bodies could increase the period and semi-major axis of one of the objects?