Where are you seeing the leaked fluid, and how much? Is there a corresponding fluid loss at the master cylinder, and how much? The master cylinder rear-brake reservoir is small, so any loss should be obvious. The only thing I can think of that might cause a caliper failure that quickly is excessive heat, such as from a dragging E-brake. They just don't fail that easily, or that often. Be certain the fluid you are seeing is actually brake fluid. An overfilled axle or failed hub seals will cause axle lube to leak onto the rotors and caliper area. An axle flange leak (very common) generally makes a mess, but could also leak lube onto the rotor area, but that should be ridiculously obvious.