Unexplained random haptics

I would suggest a couple things that you could try:

1. Using the paired iPhone, open the Watch app then click on "Notifications". Scroll down to the section "Mirror iPhone Alerts From" and give this long list a good study. I'm sure that there are a bunch of them that are on but that you really don't care about seeing a notification from. Turn a bunch of the unnecessary ones off and see if it helps.
2. I'd try to reboot both the watch and the paired iPhone. Or even try doing a Forced Restart on the watch.

Apple says it best when, in this article, they say, "If something isn't working right..."

Restart Apple Watch - Apple Support

Let us know if these things help you or not.

Best of luck to you 😉

Try turning off Haptics (from your iPhone) and see if it goes away:

Watch > My Watch > Sound & Haptics

Are you sure it is not because of a Message, Stand Reminder, or some other notification?

Watch > My Watch > Notifications

and see what is enabled.