The user was confused because he thought you wanted "mine" answered based on the title with contained the possessive "my." He went forward by merging the context and Michael's question while correcting the grammar.

In the future, I would alter the context and put your question about the grammar at the very end of the comment. Also add quotation marks "example" to indicate Michael is speaking. So the whole thing looks like this:

Title: Please, somebody fix the grammar in the interrogative sentence.

For context: There was a fire and Michael is asking about how big it is.

"Can we assume that this is a large scale incident?"

Also, why did Michael want to assume that this is a large scale incident?

I know you wanted to tell users that you had a question by indicating with "mine:" but if you add it at the end of the comment, we will know immediately that you have a question outside of correcting the grammar in the comment.