Why does my Apple Watch make random calls?

No, it made random telephone calls, one of the calls was to a Niece that I rarely call.

I had it happened to me a couple of weeks ago when I wore it while gardening (which I try not to do). I reset it it by pressing in the crown and side button until the Apple logo apppeared, since then it hasn't happened again.

Is it calling your emergency contacts? Maybe the fall detection went off and you didn't hear it?

They were responding to DanEvs.