I have installed a wooden fence in my yard using outdoor use decking screws. I've noticed that some of the screws in the boards I've installed last year are starting to snap in half and I'm having to replace them. Is there a reason why this is happening? Or is this just to be expected routine maintenance?
Fence boards tend to warp due to extreme sun exposure and changes in moisture levels. This puts a substantial strain on fasteners. If you did not purchase high-quality screws, this would explain the problem. Low-quality fasteners use weak metal alloys and have much less strength than better screws, which would pull through the wood before they'd break.
You can add new screws as needed or pull and replace them with better screws all one at a time (to avoid having to reposition boards).
You could determine the cause of failure by looking at the screws under an eye loupe or strong magnifying glass. This automotive web page includes photos of tensile, shear, and torsional failures in steel fasteners. Comparing your broken screws with the examples should reveal the kind of stresses that caused the screw to fail.
Flat-sawn boards tend to cup when they dry. The direction will be the opposite of the grain, i.e. the round tree rings try to flatten out. The cupping may be putting extra tension on the screw.
In a typical fence design, the boards should be supported by the stringers. If the fence sags/warps/bows/gets pushed, then extra shear stress will be loaded on the screws.
I've snapped screws while attempting to drive them into redwood heart-wood; its much harder than spruce/pine/fir. Even if the screw doesn't break, it may have been torqued past its yield point, which would make it weaker than usual.
Lastly, it doesn't take much strength to keep deck boards down, and so deck screws are built with mild steel and lax quality. Try a larger diameter or better grade.
Deck screws come in a few different diameters , so different strengths. Shorter screws are likely to also be smaller diameter. Ideally you drill a pilot hole in the fence frame and a clearance hole in the fence board. When installing 2x ( 1.5 " lumber ) I just drill a clearance hole and no pilot ; works fine . Worst case, with no drilled holes , the screws would be highly over stressed and only a small service load could cause failure. With small diameter screws and a frequently wet location , failure by corrosion is a small possibility. And as noted there could be quality issues.