Translation:The pigs suddenly decide to work together.
Why can this not be, "The pigs decide at once to work together".. Not the greatest English, but I feel like it is the most literal translation..
You could say they decide "all at once", which means the same as "suddenly" i.e. without any warning. Fekkezaum is correct: saying "at once" would mean "immediately" i.e. without delay, but not necessarily unexpectedly.