I have the same issue on my 01 X5. It seems to pull tight when it's hot outside and then have those bubbles in it once it cools. I suspect the adhesive on the material has failed and only the expanding plastic underneath tightens it up once it's hot.

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2013 X5 xDrive3.5i Premium
2002 530I Sport Package
2001 X5 4.4i
2001 M3