Maybe someone mentioned this before, but having been in a similar situation (albeit with a bit more altitude) - approaching into the sun without a contrasting background can be very misleading to your senses. You can't really see the horizon. With TOGA thrust suddenly selected, your body will interpret the acceleration as being thrust upwards and tumbling over backwards.
If the crew didn't get their eyes inside quickly to verify the climb (as opposed to just feeling it), the whole thing would have been almost over before they realized their mistake. They may have pulled back, but not enough. With a non-fly-by-wire aircraft, it pitches up automatically - but FBW stays where you left it...
The aircraft appears to have been relatively light - only 100 pax, fuel burned off. Selecting TOGA thrust must have had a significant feeling of acceleration. Combined with home-base complacency, get-there-itis, inexperience, fatigue, ... recipe for disaster. Just my 2c worth