The stress is caused by multiple cooking uses over time, which exposes them to alternating extreme temperatures. I have used mine in the freezer, oven, and dishwasher. I do not use a microwave, but this glassware is supposed to be safe there as well. It may be somewhat rare for explosions to occur, but it has happened over the years, and people have been injured.
Pyrex was invented in 1913 when the wife of the vice president of Corning Glassworks asked her husband to come up with some cookware that would be oven safe. After two years of product testing, Pyrex was born. ( history of Pyrex)
Originally, Corning made Pyrex with thermal shock resistant  borosilicate glass. Borosilicate glass is the type that is used in professional laboratories due to its durability. Corning sold its Pyrex brand to  World Kitchens in 1998. It is unclear to me where most glass by World Kitchens is manufactured, but it is reported that since World Kitchens took over the Pyrex brand, the type of glass used in the process has changed. ( World Kitchen denies that Pyrex was ever made of borosilicate.) Today, Pyrex sold in the United States is made of  tempered  soda lime glass. While Pyrex continues to claim that this glass is just as safe, it would appear that is not the case, based on multiple consumer reports of breakage and injury.
CBS13.com investigated some complaints in its  Call Kurtis segments: Part 1 and  Part 2. Multiple complaints involve exploding glassware and resulting injury. ConsumerAffairs.com reported about breakage and injury from Pyrex bakeware in 2005 in its article entitled  Bakers Beware: Shattering Pyrex Pans. Another article in 2006 was entitled  Pyrex Panic: Shrapnel in the Kitchen. They also have kept on top of the issue with a follow-up report in 2008 entitled  Three Years Later: Pyrex Dishes Still Go Boom.  Complaints on ConsumerAffairs.com are as recent as today, November 10, 2009. There have been burns, body parts pierced by flying shards of glass, tendon and nerve injuries, as well as damaged ovens and ruined food.
A  Google search yielded multiple news sources, blog articles, and complaints about Pyrex baking dishes exploding. Follow the link and read the multiple news stories from  WCCO,  Mercury News.com,  CBS2Chigago, and Mercola.com, among others.
The complaints and injuries are not limited to Pyrex bakeware. Anchor Hocking is now made from the same type of glass and has seen its own share of consumer complaints. Complaints.com is one source for these complaints about bakeware exploding while cooling.  One complaint states that the item was cooling on a wooden cutting board (not on cold or wet surfaces, as would be the case with a consumer not following mfgr directions).  Consumer Affairs.com has complaints as recent as NOVEMBER 1, 2009, and many more. One woman ended up with a shard of Pyrex Glass Measuring Cup in her cheek after microwaving it to melt butter. ( reported by spouse: Andrew of Spokane Valley, WA in 2008.)
Bottom line: It appears that our glass bakeware and cookware are not made of the same material and to the same exacting standard as was our parents' and grandparents' cookware. Pyrex and Anchor Hocking failed to send out the memo that they were changing their composition. They continue to deny any safety hazards. I chalk it up to greedy economic concerns. I had been using a lot of glassware in recent years because of concerns about aluminum and teflon.
However, I also own stoneware pottery and iron cookware. We use the iron A LOT in this household. This holiday season, I am throwing out all glass cookware that is newer than 1990, and will only be baking and cooking in ceramic stoneware pottery, iron, and stainless steel.
I wish you safety in the kitchen!
[1 ] http://www.ehow.com/about_5076991_history-pyrex-glass.html
*I am not a scientist or an authority on glass bakeware.
My opinions are my own, and are based on my
internet research, and from anecdotal consumer