I believe the answer lies in how the Automatic Flight system will behave in the event of an engine failure. It's all in your FCOM (Tech. manual) in section 4.20. system description. There it will tell you that whilst pitch is in the TOGA mode, if you should lose an engine, pitch will command somewhere between V2 and V2+20. If you have remained in TOGA after the flaps have retracted and lose an engine, then if the TOGA logic still applies it may try to command a pitch attitude to bring the speed back to V2+20 (assuming it has electronically remembered what V2 was!). By selecting another pitch mode when the aeroplane is "clean" either VNAV,LV CHG or V/S you come out of TOGA mode and the above scenario can no longer happen.
It's the same in the single engine go-around scenario, once clean you select LVL CHG, and select max continuous thrust on the N1 page of the FMC.
The reason you need to either manually press the N1 on the MCP (or wait for N1 to be automatically selected via the FMC at thrust reduction altitude) is because your previous A/T mode was ARM, after THR HLD (which is a mode that allows manual thrust adjustment without interference from the A/T computer). By selecting N1 your thrust is now set via the FMC and A/T computer, and gives you the required thrust for the phase of flight that you are in (climb, cruise or descent).
Last edited by Capt Chambo; 24th January 2010 at 21:11.
Reason: Clarity & grammar