Hi Yen Chen,
That's a great question. This is a phrase that gets used in the news a lot, or in very formal situations where people want to sound a little removed from what happened. 'Speculation' is a noun, meaning 'guesses about something without having enough information to be certain' (according to the Cambridge Learner's Dictionary). What is strange is the verb: the verb is 'to mount', which has a few meanings. In this sentence, it means 'to increase in amount or level'.
I Googled 'speculation is mounting that' and the first post is 'Speculation is mounting that Apple's next iPhone could cost $1,200 or more.' So, we can say that many people are guessing about the price of the next iPhone and they think, more and more, that it will be extremely expensive. You'll want to use this phrase always at the beginning of a sentence when you want to sound very formal.
I hope that helps!