Keefe explained his decision to go with Kallgren after the morning skate today saying, "[It's a] combination of [Kallgren] staying active and staying involved but also managing Samsonov. This is not an easy road trip for a goaltender in particular And he's been a little under the weather throughout the trip as well."
It would seem that Samsonov was going to start tonight, but given that he isn't feeling 100 percent, Keefe opted to get Kallgren an extra game on the road trip to allow for Samsonov to rest.
Looking at the Leafs' schedule, it makes more sense for Kallgren to get an extra game against a weaker team on the road trip, as once the Leafs return home, they will play 6 games in 10 days, including two back-to-backs - one of which will force them to travel to Carolina for a puck drop less than 24 hours after puck drop against the Bruins in Toronto.