I guess temperature has some bearing on this too! I was using a wheelbarrow at work last summer (with the tyre inflated fairly hard). Stopped for lunch, said barrow was left in the direct sunlight. 20 minutes later there was an almighty bang. The air in the tyre had expanded to the point where it actually burst.
In a similar way, I was talking to a guy who does dragster racing. I was amazed to hear that as he makes his way to the start line, there is only around 6-8psi in the rear tyres. As soon as he spins and burns them up to gain traction the pressure increases quickly and massively, inflating them to their correct profile. He said, the worst thing you can do in a drag car if the steering starts going, is take your foot off the gas, as the tyres start to deflate and cause all sorts of handling issues.
Perhaps fluctuations in temperature between when the tyre was inflated and when you checked it may also be contributory factors?
