I presume that it is set by the developers, and because it is a serious change then they want to make sure that most of the network is going to change. If it was just 75% then there could be alot of miners who dont agree and we could end up with 2 bitcoins
Correct me if Im wrong but we only could end up with two bitcoins if we do a hard fork. A split/fork in two bitcoins cant happen with a soft fork, right?
A fork is a fork, whether hard or soft. The difference is that, in a soft fork, a legacy node still considers a segwit transaction to be valid (though it doesn't interpret it as a segwit transaction). In a hard fork, a legacy node would consider a segwit transaction to be invalid.
The reason that there is still a fork is that a legacy node disagrees with a segwit node on how the outputs of a segwit transaction can be spent.