It's the moisture...if you're in AZ where it's hotter than hell, you still have dry air. The only way the adhesive will fail is if it cracks. However in FL, we have near the same temperatures (our heat index has been over 100 the last couple weeks) but also high levels of humidity. That humidity might make the adhesive "gummy" which coupled ith the weight of the glass and the stress points around the corners of the glass when the top is down, could lead to the failures

granted this is all speculation but my $.02