M.C.
Something has to have triggered it.
A similar incident happened with our 2-year-old with "public" eating. We threw a party at our home for my mother's birthday last year, and it was the first time my daughter met my mother's new boyfriend. It was a brunch-time party and everyone was eating. It was December and the Christmas tree was up. Our daughter would take food from her plate, leave the table, go behind the tree, eat, and repeat. We didn't think much of it - maybe some shyness around the new guy.
Her behavior continued for some time. In public, she would insist on hiding under the table to eat and would not allow people to watch her eat anything. Even at home, if we had guests she would continue this odd behavior.
She has since grown out of the weird-secret-eating-stage thank goodness, and has also grown quite close to my mother's boyfriend who was the only new face at the party at the time. He is such a great man but I think he scared her upon meeting. He wore a hat the whole time, and he was sort of loud and goofy.
I think it was something to do with a sudden change in the home environment (the big party) and the added factor of a new visitor in the mix.
I know this is not as helpful as I had intended it to be, and I apologize!
Has your daughter been around her aunt's house a lot? Has she been around the husband much?