What are the two of the most common factors leading to brake failure?

I'm going to assume you mean while driving and not that the brakes have broken. So two of the most common factors of brake failure that I hear and read of are:

1. You may have air in your lines which will drasticly change pedal feel and performance of the brakes. In this case the lines will have to be blead, you can do this yourself but if you have any doubt get a professional to do it.

2. I'm assuming you have an older car as it would be rare for a new car to have braking problems, in which case if you are running rubber brake fluid lines the lines could be expanding reducing line pressure. This is easily fixed by fitting braided lines, be sure to buy lines that will be able to withstand the pressure.

Ans 2 - Brakes are every driver's responsibility. -Check fluid frequently, check pads at least every 3 months.

The two most common causes of brake failure are inattention and neglect !