Their normal flight path would take them north from Chicago cross Elliot Lake (CYEL). You have to wonder why they had a flight plan that took them through southern ontario.
My guess is the flight plan was filed at a time when the storm front was north of Chicago; they usually leave between 5:00 and 7:00 pm; mostly prior to 6:00 pm CDT. This flight left at 7:55 pm CDT. At any rate, they got stuck having to go through a rather nasty bit of weather which may have caused the problem.
It is odd that they didn't request deviation due to weather. It would have been a grand deviation as the cell was pretty spread. They've must have hit a pretty bad tail wind to show an overspeed caution.