Occam's Razor..this is just a guess on my part but sometimes in situations where a living arrangement changes, kids, especially older ones may not be happy in the situation with a different dynamic. Maybe they want their Mom and Dad back together, perhaps they voiced this opinion and it's not well recieved.
Added to this, we have no insight regarding the relationship between AS and TS, although if I had to guess I would say there's a good chance the relationship was on the rocks. AS kicked TS out of the home when he realized something was wrong and his son was nowhere to be found. This is very telling because in most relationships there is usually the benefit of doubt, but not here. I am thinking the relationship was already in a bad place.
Perhaps Dad saw some warning signs, things that didn't sit well. If Gannon voiced his dislike for his step mom along with other red flags seen by Dad, TS may have come up with a plan so that AS couldn't just easily cast her aside because now his son is missing. A power play on her part. You know what they say about a woman scorned. Even though it's a highly immature and juvinille way to deal with a relationship gone bad, some people can be just that vindictive.