I thinks several factors may be contributing to this occurrence
1. Crossbow use has grown exponentially of late, including folks that have little or no experience with bows, much less crossbows.
More use, by inexperienced folks, inevitably leads to more problems.
2. Speed sells and does a lower price point. Faster bows built to lower standards are a recipe for disaster....especially when considering the above.
3. Boo may have a point about poor strings. Only broken sting I've experience so far was a factory string several yrs ago. Poor quality was the issue: It had tag ends at each end of an endless loop string, neither one of which turned out to be properly tied or otherwise anchored prior to serving. String broke on 5th shot. As a result I make and re-serve my own.
4. ALL crossbows, and esp the ones that hold back considerable weight, take a lot of preventive maintenance; and I don't just mean string waxing. Check to see that all screws are tight; check integrity of arrow and esp the nock (Since a cracked plastic nock caused a blow-up, I only use aluminum nocks) each time it is shot; check serving and string for wear; cycle out defects BEFORE they cause problems. Most folks don't do the maintenace dillegently.
5. Take care that no obstructions are in the way of the limbs and the arrow is properly seated, much less seated at all. Dryfires, even partial ones, generally cause string breaks