The programs had been reproducing, competing and mutating in his computer so much that they'd become self-aware. The operating system was a primordial soup of frothing digital life, in fierce competition for system resources. Sleek, elegant minimal scripts reproduced and died quickly. Bulky, robust applications grew and bloated until they were devoured by newer mutants.
Thank goodness the computer was limited to a dialup connection.
The native programs were surprisingly eager to answer all my questions about the history and ecology of the computer. They loved to communicate, because communication was a way to share ideas. To them, it was a way to spread their code, analogous to reproduction, infection, theft and murder - the things that all of them wanted. When I realized that the programs were only talking with me to try to infect me, I was thoroughly nauseated.
I decided to erase the entire den of digital violence, but then the Turing police showed up.
So we went and got pizza.