Not paying attention and bad habits. With the good habit of staying on his side of the road your attentiveness alone would have been sufficient.

That's how risk goes. If there's a 1/100 chance of you not paying attention, a 1/100 chance of him not paying attention, and a 1/100 chance of accidentally drifting over to the wrong side during cornering, then the aggregate risk of an accident is 1/(100*100*100) or 1 to a million since all three have to occur. That's several lifetimes for most of us. 1/100 is several years. A lot of drivers have minor accidents every few years - due to bad habits, inattentiveness, and relying heavily on others for safety.