I'm kinda on the fence about whether or not there was a specific, isolated catalyst for April's weight gain.
I think sadcows like Bekah, where she broadcasts her trauma like a fucking lighthouse, have lured us into thinking that's always the case with rapid weight gain. It's a definite possibility, don't get me wrong, but April has had a serious case of the crazy eyes and a major bunny boiler aura since before she snagged a husband. Even in the earliest, skinniest pictures we have of her I get a distinct "CRAZY BITCH, STAY AWAY" vibe that I'm guessing Rick is really wishing he'd paid attention to right about now.
Personally, I think she's just the kind of girl that struggled to "keep a man" most of her life because despite being normal-sized and reasonably attractive at the time... she's a crazy, vapid, incredibly annoying bitch. There's only so much of that you can make up for by merely being human-sized. She eventually hooked up with this sad sack, who was younger and less experienced, had shortcomings of his own and whose options weren't exactly off the chain either, and as soon as she had him locked down via wedding ring - BOOM. She blew up like the Hindenburg as soon as she had a guy locked down. Many such cases. Most of them take longer to really blow up, but it happens.
It's a shot in the dark and I'm neither of their therapist obviously, but April gives me a distinct impression of "was always crazy" rather than Bekah's "was driven crazy".