As far as we know, it was just a malfunction.
You might apply Hanlon's razor to the situation:
Hanlon's razor is an aphorism expressed in various ways including
"Don't assume bad intentions over neglect and misunderstanding",
"Never assume malice when stupidity will suffice", and "Never
attribute to malice that which is adequately explained by
carelessness". It recommends a way of eliminating unlikely
explanations for a phenomenon (a philosophical razor).
In other words, we have no reason to believe that anybody sabotaged ED-209. Instead, the accident can be explained by rushed development, poor planning, improper testing, etc.
You could go a step further and say that this is part of the satirical nature of the movie, criticizing business practices that favor profit over the safety of the general public, but I'll leave that as an exercise for the reader.