Why is my dog doing this? (details)

I have two dogs, and they have both been housebroken for years. The younger of the two came from a very sketchy background, and tends to be food aggressive – so I feed the dogs in their crates. I have done this since she was a puppy, so both dogs have had this routine for 2–3 years.
In the last few months, the older dog (she’s about 5) will eat and then immediately poop in her crate. The weird part? If I don’t lock the crate, she is able to open it herself… and if she can let herself out immediately after eating, she could wait hours until having to poop. This behavior of crapping instantly has only started recently, and I don’t understand it. Why might she be doing it, and how can I make her stop?

What gives??