Both the mayor and city council have proven a perpetual motion machine of criminal investigations, click here. Notably, the prosecution just filed a superseding indictment alleging that Kilpatrick had other affairs beyond the one with his former Chief of Staff Christine Beatty. It is not clear if this subpoena is linked to that expanded investigation.
The deputy has alleged that Kilpatrick (who is 6-foot-4 mayor and a former football player) knocked him into the female investigator. Any contact with an investigator is an invitation for a battery charge. The media attention will not help the situation since police may feel obligated to show that you cannot shove officers in such situations.
This is precisely why most lawyers keep clients out of locations where subpoenas and searches are being conducted. It is always a tense scene where tempers can get the better of clients. When a client must be present, an attorney should always be present for that reason.