Could it have been that your students were just bored with what they had been using? (i.e., not "anki" vs. "memorang", but "something I've used" vs "something new" effect?)
It's an interesting thought! Boredom can definitely lead to study fatigue, but the people in this example were medical students preparing for a high-stakes board exam at a top-10 institution. Boredom might be less of an issue at this level - it's more about efficiency and a tremendous feeling of stress and pressure. Indeed, several of these students who were in danger of failing were studying as much as 10-12 hours per day... essentially spinning their wheels in the mud.
That being said, combatting boredom is a really important thing to consider.
That being said, combatting boredom is a really important thing to consider.