The forum term for this is “poltergeist.“ Search for that in the forum and you will find a lot of entries. 
They haven’t been too many for the last year, but there have been some in the last month or so. Some of these were explained by platform issues reported on the status page, most typically delayed events. But some were just random.
Here’s one recent thread:
Sometimes there’s an explanation. If you use Alexa, make sure you check the routines and hunches in the Alexa App: it’s a little too easy to authorize the system to create an automation for you.
Also, presence-based automations are particularly likely to misfire if your cell phone briefly loses connection.
But sometimes there isn’t any explanation other than that the platform is acting up. 
As I’ve mentioned, several years ago I myself had an issue with a Z wave doorlock that kept unlocking in the afternoon. It did this every day for several weeks, and senior engineering staff looked at it but couldn’t figure out why. Eventually I ended up removing that lock from my smartthings set up, moved it to a different zwave hub, and never had any more issues with it. 
Then there was a community member who was getting alerts based on a “dining room sensor“ when they didn’t have a dining room! That one ended up looking like database corruption in the cloud.
Sometimes you never know what’s causing a poltergeist problem, but you can do some detective work to see if it’s anything obvious.