Sounds like you transitioned to 'on approach' logic. FCOM 2 explains when this happens ( within 2nm of an approach point including transitions ). VNAV will revert to PATH even with the speed window open. However there are other criteria which will 'force' VNAV into PATH with the speed window open. One of these is selecting flap which sounds like the condition you may have activated. If you don't have an accurate representation of your expected lateral path depicted on your NAV display then the aircraft will either pitch down (and accelerate) in order to capture a profile, or simply level off if below the profile. The solution here is to ensure you keep you lateral path updated. If not, stick to LVL CHG or V/S.
Last edited by ast83; 12th Dec 2015 at 06:15.