How about this;
The change in CW's behavior (distant, according to SW) and the closing of his FB page are markers of when his planning began. Maybe not planning anything extreme yet, but planning changes. Sometime after they get home from NC, but before SW leaves for Arizona, something happens, or is said, which makes his plans become more extreme and sets them into motion. In turn he says something to her, while she is in Arizona which causes her to not feel well and perhaps be aprehensive. The delayed flight is the final trigger, because it eliminates three or more hours he had planned on, and he probably blamed her. Now horrible things begin to happen. I believe the girls were killed first, to punish SW when she arrived home.
I also believe there are triggers, if we look for them. The first trigger was his appearance and people being more attracted to him, increasing his confidence. The second trigger was the affair. The third trigger was, coincidentally, the third pregnancy. It countered the elation of his affair and made him feel trapped. The fourth trigger was something that was said,or happened, before SW went to Arizona (Maybe she told him they needed to discuss separation when she returned). The final trigger was the delayed flight. Each trigger moved things forward and led to the next trigger until the horrible, final outcome occurred.
I mostly wrote this to get things straight in my head. And I edited it a couple of times. I would appreciate any comments.