Everyone offers good points above but I'll ad my 2 cents. First off, what was the "lb test" or "x" value of your leader, if you know? There are a lot of variables here that we don't know. How hard were you pulling, were you actually using a leader too light for the style, maybe the leader was dry rotted (I've had that happen with "brand new" leaders/tippet,) and finally, what was the integrity of your knot like?
There is a good possibility that it may have been your knot, especially given that you were using a clinch knot. If it was the regular old clinch knot, which is very easy to tie, you really have to use caution when tightening. It has a tendency to slip, especially in lighter tippet, so I recommend making more than the standard 5 turns around the standing line. The improved clinch is only slightly better in my opinion and, once again, if care isn't taken when tightening and seating the knot it can be rather weak as well. Always lubricate your knot before accurately seating it for best performance. Also, try to look into and learn a few other knots for a attaching flies. Duncan loop, trilene knot, Palomar knot, etc.