Let us understand the meaning of "over the top". It is used for describing someone's reaction or something as excessive and exaggerated (and unacceptable/stupid/offensive).
After understanding the meaning, let's incorporate the word into meaningful sentence. We can use the phrase as an adjective.
Example: We tried to pacify her, but her reaction was over the top. In this sentence, "over the top" describes the noun (her). It functions as an adjective.
In your question, "His reaction to my question was really over the top." means the man reacted (perhaps too aggressively or too angrily) when he heard your question.
There are other definitions for "Over the top". We use it to indicate that something we have done exceeded our goal or quota.
Example: Financial statements released showed that profits and cash flow were over the top for the latest quarter.
Other examples are:
The movie had many gory scenes and over the top violence.
It is over the top to label them as terrorists without proper investigation.
Hope this helps.
Cheers, Mr Lance