Why would a plane drop 14.000 ft mid flight?
I looked up the flight on Flightradar24.com. The descent starts at 16:10 UTC and ends at 16:29 UTC, going from 38.000 feet to 24.000 feet. So, the aircraft descended 14.000 feet in 19 minutes. 

14.000 divided by 19 makes roughly 750 feet per minute, which is a relatively normal descent rate for airliners. 

The cause for this is probably an ATC instruction. The descent starts around 20 minutes ahead of the border of the U.S.A and ends directly in front of it (SSM VOR is the entry point to US Airspace).



Source

So, this is probably the cause for the descent: The pilots contacted the US controller around 20 minutes ahead of them entering US airspace. The controller then instructed the pilots to descent to 24.000 feet and reach this altitude at the SSM VOR. So, no uncontrolled drop there.