She just changed out of nowhere, and she doesn't know why.

After closing the door and trotting over to set the groceries on the dining room table, Sarah walked closer to Tom and looked like she was on the first of tears. Gesturing down at her horse-body, she told him, "Tom, I don't know what happened! I was just shopping, getting some nice looks, when all of a sudden my lower body changed into this! I got so many weird looks, and I couldn't drive my car back, I had to run!" Reaching up, Tom cupped his wife's face, and brushed a few tears away with a finger.

"It's going to be okay," he told her. "Why don't you rest, and I'll go look online, see if I can find anything out about what's going on?" She nodded, and realizing she couldn't exactly lay on the furniture with how much she must weigh, had to settle with laying down on the living room floor. Meanwhile, Tom went to the computer room to mess around online and pretend he was looking things up. So, it was as he'd expected: despite the fact that none of her previous changes had stood out to her, there was no way she could ignore suddenly becoming a centaur.

While sitting at the computer desk, Tom's cock was straining in his jeans. She'd been just as sexy as he'd thought she'd be. So, she knew something was up, but it would only take another quick wish to make sure believed whatever he told her. That did bring up the question for him though: how was he going to explain this?