Because volume expansion from a reaction wouldn't be that large. Things aren't burning/etc. Direct injection of large volumes of water (with those things in it

Because volume expansion from a reaction wouldn't be that large. Things aren't burning/etc. Direct injection of large volumes of water (with those things in it) is more than enough to explain it. No need for reactions of any sort. The issue is that the injection should be into far deeper sections. Something in the active injectors is leaking and therefore the injected liquids aren't making it to the bottom of the well and are flowing out the sides and into shallower permeable sections. The simplest explanation would be a bad casing (i.e. the steel sides of the well) in a major disposal well, but it's likely more convoluted than that. Either way, it's a significant problem for a lot of reasons.