Hi Karen,
This is a really interesting question, and it has to do with the structure of the brain and the different regions being used in the task!

The first important thing to know is that sensory input from the left side of our body is processed in the right hemisphere of our brain, and input from the right side of our body is goes to the left hemisphere of the brain. So when participants received input to their left ear, those signals went to the right hemisphere of the brain. Then, to take that information and relay it into speech the participant needed to use the Broca's area we learned about in Lesson 4. Most aspects of language are primarily processed in the left hemisphere in the brain, and this is where Broca's area is found.
Putting this information together, when a signal is detected through the left ear it goes to the right hemisphere, then travels to the Broca's area in the left hemisphere, where it is verbalized by participants. When inputs are given to the right ear the signal is already being process nearer to the Broca's area, making it easier to verbalize those inputs, and making it more likely for them to intrude.

There are lots of really fascinating (and kind of freaky!) studies that delve deeper and expand on this phenomenon using split brain patients. These are individuals with their Corpus Callosum either partially or completely severed, and the hemispheres of their brain are unable to communicate with each other. Well worth reading about when you've got some time!

Nicole