What causes a tire to lose air only when driving at 70mph?

I have a 2012 ford focus and the rear driver side tire loses air to the point where it's flat when driving at 70mph. The tire never did this before. One day I went on the freeway like I always do and was driving at 70mph, and suddenly I got a low tire pressure warning and about 15 minutes later I stopped at a gas station and the tire was flat. I filled it up with air and drove on it on normal streets (not the freeway) at about 45-55 mph and it didn't lose any air, the car is currently parked and it still hasn't lost any air since then. What could the problem be?

Total guess - there's a dent in the rim and the heat or force applied at higher speeds causes the tire bead to slightly pull away from the rim.
Second guess a bad valve stem that leaks a higher pressure.

The problem could be you not taking the tire to a shop and having it looked at.

Check the device that tells when air pressure is low for air leaks. It could allow air to escape when driving.
Otherwise, you either have a tyre that has a hole and will leak all the time, or you have a bad tyre that is unsealing when you drive at speed. That is improbable.
You could also check the wheel for cracks, maybe there's a crack that will pass air when moving. That could be dangerous. Also improbable.

If it was low already before you drove it faster, then that may be the reason. Faster speeds will make the tire pull away from the rim. If there's not enough tire pressure in the tire when accelerating, then it could cause the rest of the air to leak out.

Test this idea by first checking your tire to make sure it is fully inflated. Then drive it fast on the freeway to see if it will lose air again. If it does not, then it lost air the last time because it was already too low! Like maybe 10 PSI or so! How it got that low is whether you have a slow leak or someone let the air out on you!

But If it does still lose air on the freeway after you start with a properly inflated tire, then I'm wrong and you need more answers!

Hi so what do you expect tires don't last for ever sunlight destroys the synthetic rubber the tires are made of. The tires will be cracking and showing signs of wear.So time to have then replaced. The last thing you want is a blow out at 70 mph.Death is likely to follow.

Have you checked yet for a puncture? A nail or screw can cause a leak that only leaks occasionally.