Here's my thoughts on what may have happened...this is just my opinion.
Bob loses his job and takes a construction job 5 hours away, now I'm assuming he stayed somewhere close to that job. This now throws all the responsiblilty on Gayle who is trying to work, take care of the kids, the bills and everything else around the house. Bob now feels like he's not the bread winner and he's not taking care of his family like he's supposed to...Gayle is making more money and his ego is bruised at this point. Some time during all of this, he comes in contact with another woman who sympathizes with him, consoles him, says all the things he needs to hear. The affair begins. Now when Bob is home he thinks about the other woman more and more...Gayle is stressed out about handling everything but doesn't want to argue about it, Bob keeps things stirred up and the arguments going because this gives him a reason to go see his new love interest and put the blame on Gayle. In his mind he's blaming her for his failures and for pushing him into the arms of another woman. He and his new lover want to be together. On the weekend Gayle disappears, Bob and his lover had already planned to be together, but Gayle invites her relatives over and messes up Bob's plans. He can't leave home when he and lover had planned so lover calls his house because she can't get him on his cell. He lies to Gayle and her relatives about who was on the phone. Gayle already senses something is going on. After relatives leave, Bob is furious at Gayle for screwing up his plans. The kids go to bed, Bob gets Gayle outside to argue, he chokes her and then puts her in his pickup and takes her away. He is so angry and so focused on getting rid of Gayle that he forgets about his kids being at home alone. He gets rid of Gayle then drives to be with his lover.