W/O seeing the problem, it's hard to tell. Beings that the problem is random, you may have some bad bricks. It's possible.
Do an inspection to see if water is getting behind the bad bricks from above. If so, its freezing and thawing could be causing the problem. I've never had to replair or replace just 1 brick so I can't advise you on what to do. Perhaps there are others who can say something more.
My best to ya and hope this helps.
PS: While you have the ladder out, look around at other bricks and mortar to see if you see any cracks. Inspect and monitor them to see if they lead to the same problem.