Preface, I absolutely do not condone dumping of animals be it domestic or so called 'relocated' wild animals...
That said, sometimes animals simply wander off and decide to become feral... I saw peafowl mentioned above, they are notorious for 'wandering' and looking for a new home, they have a 'Lets see if the grass is greener over their attitude' most of the time... That isn't to say all will leave home but it's not uncommon especially during mating season where they have been known to wander many miles chasing down a mating call... Same with guineas, they can be hit or miss and go feral just because, especially if you live in a heavily wooded area...
There are also instances where people die, I'm not saying it's right but sometimes when people die if someone else isn't there right away to care for the animals and they are able to wander they very well could wander and become feral... Plus there is the whole physiological aspect, sometimes people just fall apart and sad but true the animals almost always take 2nd fiddle to peoples health or needs...
I now the common response is simply 're-home' them but the reality is it's not always that easy...
I saw this poop storm first hand, my llamas came from a home where the women's husband died and the property went into foreclosure, the women loaded up her car one night with a few things and disappeared, left all her stuff in the house behind and left the animals behind ... The neighbor was taking care of the animals and working with the bank allowing here to continue to use the barn until she was able to find homes for the animals and/or locate their owners... But, the bank only granted here 90 days of barn use, and she did not have the ability to care for all the animals and was trying to give them away, but there were few takers... I only took the two llamas but I could have walked away with a half dozen Arabian horses as well... Moral of the story, I don't know what happened in the end, but it's not always easy to give animals away and people do crazy things sometimes...
Again, I'm not advocating or supporting people that dump animals, there is absolutely no reason to do it, but sometimes poo-poo things happen and things spiral out of control leaving people to do stupid and irresponsible things...