Hours "reminders" are a type of "notification," using Apple's terminology. Some helpful explanations from Apple about notifications on the Apple Watch:
If you are set up to receive Hours notifications on your iPhone, by default they will appear on your Apple Watch or your iPhone, depending on which is most available to you (see first Apple article above).
When you receive an Hours reminder on the watch, if stopping a timer is an appropriate action for a reminder, you will have the option to tap to stop the running timer right in the notification.
If you are not receiving Hours reminders on your Apple Watch, make sure notifications are allowed from Hours in your system settings (Settings > Notifications > Hours > switch on Allow Notifications) and that reminders are turned on in Hours on your iPhone (Gear > Reminders > switch on Send reminders).
If you want to stop reminders from appearing on your watch, but still want them on your iPhone, open the Apple Watch app on your iPhone, then Notifications > Hours > Mirror iPhone alerts > turn off.