I can't find 唔通 in ABC Chinese–English Comprehensive Dictionary (2003), probably for it's Cantonese. 唔通 (m4 tung1 | ) : could it be that ...? - CantoDict
通 means nothing is blocking and we can go through it. It applies to thinking as well. Such as 諗得通, 講得通, 睇得通 meaning there is no longer blockages in thinking and everything piece of a puzzle is solved and we know the answer and the reason.
唔通 is the opposite. There are some blockages in thinking and the puzzle is remained unsolved.
Using negation to ask a question is nothing uncommon. It is just like "Isn't it that ... ?" in English. It is the same for 唔通.
When one asks with "唔通", he is in a situation with some evidences and he realizes something but not so sure.
個天突然間黑晒。唔通會落大雨？ The sky turns dark suddenly. Could it be heavy rain soon?
Or he meets a ridiculous situation and he thinks of a ridiculous reason and ask in rhetoric tone.
佢殺咗人都無事。唔通依度無王法嘅？ He killed a man but he got no penalty. Isn't it that law doesn't exist here?
Both involves of reasoning with an answer. That is 通.