Easter candy eggs are so boring. They just sit there...until you make them hatch!
When microwaved, this Cadbury egg cracked open, letting the filling spill out into a puddle. Since microwaves work by making water molecules vibrate and heat up, the water-based fondant filling might heat up faster than the chocolate, which is why the fondant melts its way through before the entire shell collapses.
As the filling cools down after cooking, it hardens, so that instead of a sticky liquid it becomes a soft pasty candy that you can pick up (and eat if you really want to).