I had the same problem a few weeks ago and called apple support. The solution is as follows:
Most apple users have multiple devices using icloud such as an iPdad, iPhone, and other computers. In my case, I had two computers, two iPads, and one iPhone. After speaking to apple support, I realized that one of my ipad's setting were set to delete events older then 2 weeks. In short, if you are expeirienceing problems with ical deleteing events, you better check all your devices. If just one of them is set wrong in your preferences, you will lose past events. Even check computers you gave to your cildren or other members of the family. You will probably find out that the settings on one of these machines is causing your problems.