Situations
The will seems wrong
A will can only be set aside for specific reasons. These are the ones that come up.
Mental Capacity
They were no longer capable of making a will — they couldn't hold what they owned, who their family was, and what the document did in mind together. Usually the most straightforward ground to prove, because a failing mind is visible: the barber, the neighbour, people at church all saw it, and none of them care who inherits.
Undue Influence
They may have been capable, but someone else's wishes ended up in the document instead of theirs. Harder to prove — pressure applied behind a closed door has no witnesses, so the case must be built from circumstances.
Improper Execution
Texas requires specific formalities. A bad signing can void an otherwise fine will, regardless of anyone's state of mind.
Forgery & Fraud
The signature isn't theirs, or they were lied to about what they were signing.
Most contests plead more than one
Capacity and undue influence travel together, because diminished capacity is exactly what makes someone vulnerable to influence. You don't have to work out which one describes your situation — that's what the first conversation is for.