The compound that produces heat in peppers is capseicin.
What makes radishes hot?
Maybe it’s the red. I’ve always found them sexy, too.
Maybe I am missing the point of the question, but I have found that in my own garden radishes planted early in the spring are much more moderate than radishes planted later in the season using the same seeds. Just ancedotal, but in my admittedly limited experience it seems cool and damp gives fairly gentle radishes, hot and dry gives much hotter radishes. I think it because the radishes that grow much slower in the hot and dry have more time to build up the hot agents, but I got nothing to back that up with.
YMMV
Chemical reasons, not a clue.
Great Wiki link, Q.E.D. It answered my questions about hot onions, too!
**
Seenidog**, that’s been my experience growing radishes, too.
FIRE!
Similar chemicals are contained in mustard and horseradish. Someone on here did a Staff Report on that very subject.