Insurgents in Iraq and Afghanistan routinely fire RPG's at high elevation angles so that they come down and explode like mortar rounds. By luck or by design, the 4.5 second timer on the RPG round kicks off just as gravity has pulled the round back down to Earth when fired thusly.
It could be a variant of the RPG-7/14 with one of the new tandem top attack warheads. I believe this is why they created the armored vents for the M-1s engine last year as they were vulnerable to top attack warheads. This is the new trend for the rest of the world(we've been using TA warheads for awhile), so hopefully the U.S. Army will come up with a countermeasure.
If indeed and RPG-7 did it, it was probably something like this and I doubt they were being lobbed like a mortar round...probably direct fire from a location on a roof or some structure.