It might stem from the stress of a few things, and taking the easy way out socially.

-Mormons tend to have more kids and to have them sooner. This in and of itself is a strain that needs to be experienced to be understood
-Mormons tend to have high standards for themselves professionally and ethically. This can increase productivity and dependability but also comes at a cost of wanting to leave when work is done and separate oneself from work altogether.
-Mormons have a job to do at church and tend to shop and recreate less on Sunday, creating a double stressor of having a bit less time than those who don't engage in community service on top of wanting to maximizing R&R the other days of the week.

As for the easy way out:

-Mormonism is a big enough investment that it tends to change the investor. Each Mormon gets changed such that they are so easily able to relate to others moving in that same direction. It can take a bit more work to venture out into a less familiar social situation where to topics aren't as well known. I don't know that that is a valid excuse, though.

-In my opinion, the drink argument has all the credibility in the world for someone who has had prior issues with alcohol. Otherwise, I'm not so sure it's a good excuse.

In short, I suppose I am agreeing with you and offering a few explanations why. I don't claim they are right or wrong; they just are.