I bought a bag of Brussels sprouts that looked fine but smelled exceptionally bad, even for sprouts I recently bought a pre-packaged bag of brussels sprouts. The sprouts in the bag looked great, green and plump with not a spec of black or brown on them. The next day I cut open the bag and found that the entire bag smelled like a mix between vinegar and over-ripened cantaloupe. If I cut open a single sprout the smell was even stronger.

There was no evidence of spoilage or a mushy texture, all the raw sprouts were firm.

I have eaten and cooked brussels sprouts many many times before so I am familiar with their normal cabbage-like or sulfurous scent. These sprouts were not like that, it was a strong sour smell.

I tried cooking with them and the smell never went away after either boiling or roasting. They also tasted like they smelled.

Does anyone know what could have happened? Was there some kind of chemical involved?
What you are describing is what we call a food that has "soured". (This is what I was taught growing up.) The food looks and feels perfect but has that awful sour odor. 

In my experience it is from improper storage, usually from foods being kept too warm or not getting enough air circulation. (Not necessarily by you, but somewhere along the transport chain.)

I have found it in both Brussels sprouts and broccoli. I have even bought broccoli, used some of it, and later taken the rest out only to find it soured. Or bought Brussels sprouts and kept them for a couple of days before using and found that they had soured. This says to me that the souring process had already started.

Additionally, I have had it happen with fresh corn that was left on my porch in warm weather.

I have also found it in bagged salads and spinach. With those, a good washing and fresh air sometimes gets rid of the odor - but not always.

Unfortunately, we don't always know how foods have been handled and stored before we buy them.

I have searched extensively to find out more about this, to no avail. I can find information about how to tell if food is going bad or rotting, but this is a totally different thing. You have no way of knowing that this souring process is happening. That is, until you smell it.