[eureka]

I guess, force closing of WAF app is the memory management.

WAF app is essentially a html page rendered by WebKit engine with a thin layer of Kindle-specific API exposed through special JavaScript object. When you execute your own JavaScript in context of your WAF app, you can create memory leaks. Though JavaScript implementation provides garbage collector (i.e. automatic memory management), it's possible to create memory leaks in JavaScript program. With many memory leaks (or with one repeated memory leak), memory consumption grows, and at some point your WAF app will be terminated, because on terminating all memory consumed by [any] application will be freed.

You should check free memory size with free command at the start of your app and at the end of stress test to confirm you hypothesis.