I came across a quote about pregnancy that may provide some insight for expectant fathers on why pregnant women may not seem themselves.
"Life is tough enough without having someone kick you from the inside."
Pregnant women just don't feel themselves and strange things are happening. I'd be irritable if I got kicked in the stomach all the time. Aside from that, consider even some basic logistics; they're getting large and immobile, they have to pee every 5 minutes and don't have the energy to walk to walk to the bathroom. That alone would drive me nuts.
Check out some more pregnancy quotes.