Actually, jailbreakme did the exact same thing. And I don't think anyone really knows the problem. Technically, multitasking isn't really broken. You can use apps such as Skype or a to do list that gives you a badge due to the multitasking ability, yet, it seems to delete the memory of apps after you use 3-4 different apps. (Yet ones such as ToDo or Skype still work no matter how many other apps you open)