My best guess, is you have a bad strand in there somewhere. A wire is exposed or shorting out or some such thing. That is causing the fuse to blow.
To answer your second question... it depends on the quality of the strands. If you put cheap strands without sealed sockets or lighter wire, they won't last as long (not saying you did this). But, you also have a second bigger problem, and that's you have lights that are installed all year long. So you have a much higher risk of all sorts of things causing you problems. Birds, squirrels, teenagers, general man-handling, wind, sun, snow,.....etc.
Lights are typically rated in 90 day seasonal use. So, in your case, two years is like 8 years of seasonal use.