A will may be found invalid because someone in a “confidential relationship” with the decedent “actively procured” them to write a will which does not reflect the true intention of the decedent.
Elderly or infirm individuals often become dependent upon a relative or caretaker, and sometimes the relative or caretaker takes advantage of that dependency and convinces the person to make a will leaving all or a significant part of their estate to them, while leaving out close family members.
There may be evidence of some mental decline such as dementia or Alzheimers, which made the elderly person an easy target.
Every case involving undue influence has its unique facts. However, all undue influences involve someone accusing someone else of manipulating the decedent into making a will that favors them, and disfavors others.