Why did a glass ashtray spontaneously fracture into 4 pieces?

[abstracted6]

We have an ashtray on an elevated table in our balcony.

No one was outside and it suddenly fractured into 4 nearly even pieces on the table. Strangely enough, all the cigarette buds were lit.

This is the ashtray we used:

http://www.walmart.com/ip/Anchor-Hocking-5.75-Glass-Ashtray/17693180

My question is, how do you think this thing suddenly broke with all the cig buds lit?

 

[tiny-tim]

hi abstracted6!

this is an extremely important scientific topic, so naturally there are already threads on it at PF …

see eg https://www.physicsforums.com/showthread.php?t=589623 and https://www.physicsforums.com/showthread.php?t=69198

(you could also try googling "spontaneously shatter")