Many people bring a child into their home through divorce. This is a lifelong commitment to help raise that child and help them become productive adults. You should remember that this continues even if you and your spouse decide that you are going to divorce. Unfortunately, going through their adoptive parents' divorce can be a very traumatic experience for the children, especially when they are older when they were adopted.
The reason why adopted children might have such a difficulty is that they thought they had found their permanent home. It is hard for them to understand that their parents getting divorced doesn't mean that they are losing them. You will likely have to help them realize that you and your ex will both still be their parents.
It can also be hard for them to adjust to having two homes now. They crave stability that comes with an adoptive family, and they are likely concerned that they will lose one of the homes. Finding ways that help to ease the adjustment between homes can be beneficial.
One challenge that you might have as a parent is working with your ex. You need to be a team so that you can do what is best for the children. This requires you to have open and respectful communication. Trying to set standards that can span both homes might also help them to adjust to the new way of life.
You need to have a parenting plan for all children, including those you have chosen to love through adoption. Getting this set as early as possible can help the kids to adjust so they can thrive.