One of the reasons why someone might challenge a will is if the person who created the will was under the influence of another person. This undue influence might have led the person to make provisions in the will that he or she wouldn't have made otherwise.
One of the reasons why this is such an important factor is because the will is meant to relay the decedent's wishes, not the wishes of someone else for the decedent's assets. When undue influence is a factor, a will challenge is very possible.
What might lead to this type of situation?
Oftentimes, a person with unscrupulous intentions will get close to a person he or she thinks is vulnerable. They do this in an effort to get what they can out of the person. This sometimes means being manipulative to the point that the vulnerable person, who is likely elderly, changes his or her will to include the manipulator.
Who might fall victim to this behavior?
The manipulator will usually prey on a person who has recently had a tragedy occur. This could be the death of a spouse or child, a divorce or the diagnosis of a catastrophic illness or injury. The victims are usually senior citizens and they are usually well off.
Anyone can be a manipulator in these cases. The person might be a relative or caregiver. This relationship doesn't really matter in these cases as long as there is an element of undue influence. These cases might be difficult to prove, but doing your best is necessary since the fate of the estate hinges on your case.
Source: American Bar Association, "Psychological Aspects Of Undue Influence," Ira Daniel Turkat, accessed Aug. 17, 2017