This is becoming much more common than before. Specifically, I was told at Yale that almost every student at this point takes a fifth year for research or a masters degree. It probably depends on the school, but any school that offers a free fifth year will probably have a high number of students do 5 years. Also, having a lot of research done during med school may make you more competitive for an impacted residency.
It's definitely not due to academic failure, that's very minimal. Another reason, but also rare, could be illness.