Usually a bead leak. Once these type of tires get low past a certain
point, they leak even faster. Try some slime or other such tire sealer.
And don't be afraid to use it liberally.

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Slime doesn't work as it only cover the low areas inside the tire and not
anywhere near the sidewalls or the bead. Plus it doesn't really seal the
tires long term as it ooze out in time. Others may have better luck but
that's my experience.

My bead leak was caused by corrosion in the rim. Used a wire brush on an
angle grinder then follow by a good primer. Seems to work - three weeks now
and holding. Oh yes, had to remove all that slime in the tire first.