Can someone explain the reason why some aircraft pitch up while some pitch down?
Per aircraft type, perhaps. Generically, no.
There are many configurations, and therefore differences in effect and behavior of aircraft resulting from flap application. Type of flaps, tail configuration are main factors. The double slotted flaps of a Twin Otter will allow so much pitch change that it is possible to touch the nosewheel down before the mains while landing - I've seen it done, it's not pretty. It usually results in what's called (Station 60 damage). The Cessna 310 or DC-3 on the other hand have split flaps, which do lots for drag, but little for lift, and therefore have a much lesser effect on the pitch of the aircraft.
Anticipating the effect of flap application while flying is excellent, but it will not carry over from some types to others, so just learn the nuances of each type.
I have not flown the Z 701 since 1984, and that was serial number 1. It had an unsatisfactory flaperon arrangement, which I believe was changed for the subsequent models, but I never flew one.