One of the most areas of Texas Family Law undergoing the most change is Grandparents Rights. In general, Grandparents have limited rights to their grandchildren. For most Grandparents, that is not a problem - they work out things with the kids and grand-kids just fine.
However, sometimes grandparents don't have great relationships with their children and thus have to resort to filing a lawsuit for visitation and/or custody.
The 2 most common situations for Grandparents to get involved in custody cases is a lack of any visitation and really bad parenting.
The lack of visitation grounds is called a "Suit for Grandparent Access & Possession". There are a number of requirements, but basically the Grandparent(s) must prove that child's physical health or mental well-being would be significantly impaired without Grandparent access. This is an interesting burden of proof on the Grandparent, because 'significantly' is a strong term. All but the worst Grandparents would be a 'plus' in a child's life, but the real question is if that is a significant plus.
Most cases don't have these factors, and so these cases are hard to predict and prove. Courts are torn here. The Court wants Grandparents to be in their grandchildren's lives. But Courts know that litigating over those rights can poison a family relationship across the generations. It is hard to understand why a parent would be denying their own parent(s) the right to see their grand-kids. "What is going on?" is the question that the Court will likely ask.
In the Grandparent access cases that I have handled, the problem is usually a new spouse. For example, there is a contentious divorce. Dad's parents get to see the kids when Dad has possession, but Mom won't let Dad's parents see the kids. There is no case for the Grandparents here, because they see the kids. But then , imagine that Dad remarries a woman who does not like his parents. She makes Dad stop allowing the kids to see his parents. Then, the Grandparents would have the right to file a suit.
There are also situations where both of the parents are so unfit that the grandparents can file suit for custody, or even to terminate the parents parental rights. This requires very bad facts, and for both parents to be a serious problem. Even if a grandparent is granted custody, it could be that the Grandparents get temporary custody, until one of the parents gets their life back together.
I've handled a bunch of these cases. Usually the Grandparents get custody, first temporarily and then at the end permanently. Sometimes it is just temporary, for example when one parents has an out-of-control drinking problem that eventually gets better. In other cases I think that the parents did not want to be parents, and were secretly happy that the Grandparents got custody.
A note here on money. As I've said, being able to outspend your opponent in litigation is a big plus, and can often mean victory. Usually Grandparents are able to spend more money than their children can on one of these cases. Older couples tend to be more financially secure and have more savings than a younger couple. In those situations, they can often get what they want just by filing and making it known that it is going to be very expensive litigation.