I'm with NSF on this;
1.) An aircraft will always want to weathervane into the relative wind (ie pitch down into an updraft)
2.) It would seem logical that the only circumstances under which an aircraft could be pitched up and yet remain in level flight would be;
a.) in a down draft or,
b.) in slow flight,
the latter of which would also be symptomatic of an aircraft trying to stay level in a downdraft.
Perhaps there is some other phenomenon taking place in these pitch-up events that does not fall into the updraft / down draft category.