If you know Computer Science or have taken a Computer Science course and have watched all of Digimon season 3, you will probably understand what I'm talking about and possibly have already come up with the theory yourself.
I'm going to pretend that whoever is reading this doesn't know a thing about computer science or programming whatsoever. I am going to try to explain it the best way I can. This is to maybe better understand how the D-reaper grows and gets out of control.
When trying to program on a computer the object is to try to make the computer do what you want it to do by giving it commands. Sometimes what you think the program will do and what it actually does are two totally different things. It's happened to me many times in Computer Science class that the computer didn't do what I thought it would do or the program didn't work the way I thought it would. It's basicly trial and error. The more experienced one is the less mistakes one makes and the chance becomes greater that the person knows how to fix the problem. Sometimes you get the computer to do what you wanted it to do, but it also does something else that you didn't expect. This is exactly what I think happened when Shibumi was tampering with the program and trying to make digimon living creatures that could think on their own. He got what he wanted, but got something unexpected: the growth of the D-reaper. Knowing that these are real life creatures the program was probably extremely complicated. Since it had never been done before, no one had expierence on how to fix the problem. It would probably also be dangerous to experiment fixing the problem since doing so could effect real life and might lead to something really disasterous. Therefore, the programmers would possibly have to come up with a plan or action that they know is safe before proceeding.
Oh...another theory: It's all Shibumi's fault.
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