Why would master student spend more than the normal duration to complete master degree? I am looking at a particular research group for graduate studies. The research group is in STEM.

From talking to the professors, they have repeatedly told me that a master degree merely takes two years. First year you take courses, second year you complete your thesis. Sounds about right.

But when I look at the CVs of the students, a lot of the students seems to be graduating way past this two year limit. One student was doing his masters from 2011 all the way till 2014. Another recent student is still completing her master that was started from 2013. I have only sampled a few students, they might not be representative of the entire research group.

Still the question remains, what could be the reason that a master student is taking so much longer to complete his degree given that the research group has explicitly told everyone that it only takes two years.
Here is one more possible reason: the students may be working so much "on the side" that they need more semesters to complete all of the necessary courses.

Not at all universities are students supervised thoroughly or progress is checked regularly, so that working part time and studying part time is possible even when not explicitly studying for a part-time degree.

It is not difficult to imagine that when working 50% of your time and studying 50% of the time, it takes twice as long to complete the course of study. 

Obviously, it depends on the rules and regulations of the university and/or course of study whether this is a possible explanation. As an example, my undergrad course allowed this, and the average duration of study was 85% above the normal duration of study, mainly because so many students studied while working. It was a STEM course. I know of on case where the student actually spent 2/3 of the time working and 1/3 of the time studying.