Tyre Has Randomly Deflated Day off work so I didn't use my bike today, left it in my garage with nothing there to cause any damage.  However later today I walked into my garage to collect some supplies and a loud bang followed by a hissing sound (air going out) came from inside. When I checked my back tyre had lost all air in it. Does anyone know why this may have occurred?
Likely something under tension failed which compromised support of the tube leading to the explosion.  

Three main culprits come to mind:


Rim Tape - Rim tape can fail as it gets old. Usually what happens is that it can no longer keep the tube out of the spoke nipple wells.  When this happens the tube will expand into the spoke nipple wells where there are all sorts of sharp bits that can lead to a puncture. Look for a slit in the rim tape over one of the spoke nipple well holes.  The tube will have a puncture on the side facing the rim. 
Compromised Tire Structure - Your tire carcass may have become partially compromised (e.g., damaged side wall) and over time this slowly broke until the tube exited the compromised area and ruptured. The tube will typically have a large slit. 
Temperature related blowout - Is our garage hot?  If so this will increase the pressure in the tire.  If you were already at the upper end of allowable pressures (or the tire/rim combination is not good) then this could lead to and over-pressure blowout where the tube escapes at the tire/rim interface. The tube will typically have a large slit on the side (as it would have escaped at the tire/rim interface.


Inspecting the tube will give a lot of forensic information. Make sure you note the orientation of the wheel and tube as you disassemble this will make it easy to trace back to the source of the puncture.  A puncture on the inner diameter of the tube (i.e., the side that faces the rim) will point towards (1).  Large slits in the tube are suggestive of (2) or (3).  If you inspect the tire and find no damage then this is suggestive of (3).