Apple Watch keeps pausing my outdoor walk

In watchOS 5 or later, when your Apple Watch senses that you’re working out or done working out, it taps you on the wrist and asks. If you don’t respond to the notification or you dismiss it, your workout continues until you choose to end or pause. For more info, see:

Use the Workout app on your Apple Watch - Apple Support

Note: You don't necessarily have to stop your walk to get this notification. The watch takes into account several factors such as decrease ins heart rate, pace, etc.

Unless you use "Other" or "Yoga" workouts, I have heard that, according to Apple Support, for the activity ring to register your Outdoor Walk workout you need to maintain a speed of approximately 3.3 mph or faster and increase your heart rate to approximately 75% of your High Heart Rate (from the iPhone Watch > Heart app). This definition of a "brisk walk" is to encourage you to put more effort into the walk, (For an Outdoor Cycle its is 9.8 MPH (or 16 KPH) and 75% of your High Heart Rate.) If you slow down or pause, the watch will stop giving you exercise credit, It will still measure your steps and calories burnt but will not give you any credit towards the exercise ring or workout app. A brisk walk speed is different for everybody as the watch takes into account your age, sex, weight, and heart rate which is why it is important to calibrate the watch and have the correct details set up in the Health app,

I can't vouch for any of those specific numbers, but I'm sure there is some kind of criteria you have to accomplish for a "minute of exercise" to be counted as an "exercise minute. See the "Make sure that you earn Move and Exercise credit" section in:

Get the most accurate measurements using your Apple Watch - Apple Support

You should also make sure your watch is calibrated:

Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support