Cheap tube combined with tyre creep probably - mark the tyre and rim with a marker pen and, if it happens again, see if the tyre has moved on the rim.
If it has, then either use more pressure or fit a different tyre. Use talc inside the tyre too but not on the beads, of course.
If it has, then either use more pressure or fit a different tyre. Use talc inside the tyre too but not on the beads, of course.