In my last few game sessions, my players have begun adopting a strategy which is working wonders in the campaign we are playing. In this Planescape campaign, we are on the Outlands, which has a lot of open terrain in place of enclosed dungeons. As a result, we are experiencing something a little out of the ordinary for D&D, which is large open encounter areas.
The players are using a strategy that exploits the fact that reloading a ranged weapon in D&D can be done while fully mobile and with no penalties for shooting while/before/after moving. Quite simply, over the course of levels 1 to 6 they have all acquired the Dash feat and put lots of specialization into weapon proficiencies with a bow. They have been able to level encounters on the Outlands without taking a scratch by simply running away before or after shooting. Their movement exceeds most average NPCs, making them effectively archer cavalry minus the horses.
I am conflicted about this one.
On one hand, their strategy is pretty effective and quite clever. But on the other, it's making combat long and predictable.
I am not inclined to forcing the players to change their strategy. I am curious about ways I could throw in some curve balls at them without being a "screw YOU" DM. I want to break the monotony without breaking their strategy completely. Essentially, I want to take responsibility for this situation as the DM.
What can I do to break the monotony without obviously countering them? We are pretty tactical players. We like combat mechanics. Anything with a solid reflection on combat mechanics in your answer is really welcome.