When during the year did you visit? If it was during hockey season, and if they ask, say you are a Habs fan? If during summer, some street fair/festival? Keep in mind you don't have to limit your answers to just visiting Montreal. You could be visiting Ottawa/Quebec City or connecting to a flight to somewhere else in Canada with Montreal being port of entry.
Of course don't outright lie and say you visited frequently because of business meetings or you have a client in Canada if you have always marked Personal on your prior declaration forms.
Does CBSA ask you now why so often when you visit? I think their primary concern if they see visiting often as an issue is to probe whether you are working illegally in Canada or smuggling (currency/drugs/duty evasion).
When they ask a question, the answer itself often times may not be important as long as it is plausible. How you answer it and body language is what they are paying attention to. Case in point. When I come back to US from Canada, CBP sometimes ask me of my purpose of visit to Canada. My answer is really irrelevant, as a US citizen, my admissibility into US is not being evaluated. CBP is just looking for risk indicators from how I answer to determine if secondary inspection (for smuggling) should be performed. One time, I told CBP I came up for a Habs game against the Rangers (I did watch part of it in a bar a couple of evenings prior to flight and later that evening the cab driver told me who won). The questioning then stopped as CBP agent tells me he is a Rangers fan and waived me thru. LOL.