Erratic flight... can't figure out what happened

Posted a .BIN file to Dropbox:

The copter flew fine for quite a while and then suddenly started rocking erratically. Anybody have any insights as to what might have happened?

This is tagged Copter 3.4 but the log says 3.5-dev so it may be worth re-labelling this.
I’m following with interest: I haven’t found a specific problem either. You switched out of guided very quickly after noticing the rocking, so I suspect that the time period with useful data is quite short.

I’m probably reading too much into the logs but here are a couple of things I found:
Looks like the rocking behavior happened right after the copter started pitching up (to slow down?).
Right before the rocking starts, pitch dips below desired pitch (ATT.DesPitch in the logs) for about a second at t=1271s
Right after that there is a drop on motor 4 (RCOUT.C4) (to compensate for the dip in picth?) followed by a drop and a large spike on motor 3 (RCOUT.C3). Maybe a patch of rough air or some turbulence from the front propellers that propagated to the back propellers during pitching?
Have you experienced anything similar in your flights since then?