Hi,
Actually I can only guess what would have been the trigger(s). What I did differently compared to the four previous years:
1) the animals were kept cooler in summer (max 20°C)
2) we had a long and fairly cool winter (for Belgian standards) resulting in temperatures between 2-6°C from November until April
3) the animals were kept separate during fall and winter (because of the severe aggression of the male)
Actually, the newts are only sparsely fed, almost exclusively with worms. The aquarium is ridiculously simple: 90x40x30 cm³; just 15 cm of water, gravel for substrate and two (modest) rock piles (providing two "territories"). No aeration, no circulation. Water quality (nitrite, nitrate, ammonium and pH) are checked once every month.
So, basically nothing spectacular.