Disappearance of Canada Goose and Mallard colony - normal?

20 or so Canada Goose and  around 20 Mallards have left my local nature reserve lake rather suddenly (plus a solitary swan). There is often  excessive disturbance down there mainly by fishermen and dogs off the lead and in the lake.

Is leaving a lake to find another water body normal behaviour as described or could it be indeed due to disturbance? The lake itself is of good water quality and has two islands too for roosting. Could migration be a reason?

I am interested in opinions as I am a warden at the site and would like to report some expert opinions to the committee.

Many thanks indeed for reading.