There are many reasons to contest a Will:
Generally, the problem will be that the person did not know what they were signing. Perhaps someone tricked an individual into signing the Will. Or, perhaps the individual knew what he was signing but someone physically forced him to do it or otherwise pressured him until he gave in and signed it. It can also be possible that an individual signed the Will while under a delusion that makes the Will invalid.
Trusts are similar:
Most of the cases involving a problem with a Trust are due to a Trustee not doing what he or she is supposed to do. That does not mean that a Trust cannot be challenged for the same reasons that a Will can be challenged. It can. A trust can be revoked for the same reasons that a Will can be contested. Additionally, a trustee can get into trouble for dishonestly distributing trust assets or failing to take care of the trust assets according to the terms of the trust. If the trustee is acting improperly, the probate court will order the trustee to file an account and to explain his actions. The trustee may have to return money to the trust, and the court may order someone else to serve as trustee.
Mediation is an alternative:
Sometimes the parties need to mediate. Mediation allows the parties to come up with their own solution and to put it into a settlement agreement of their own. It is appealing because the parties can avoid tearing each other to shreds when they mediate. That is a good thing when the parties are related and are going to see each other every year at family holiday celebrations for the rest of their lives.