There are lots of factors that might get a turtle to wander, I'll bring up just a few.
Territory. For sure some turtles are territorial, and don't like other turtles in territory that they might claim as their own. In my case with Western Painted turtles, I have seen the larger, more aggressive turtles threaten other turtles by doing a particular technique of getting in their face and waving their front feet at the sides of the head. They don't try and bite them or anything, they just get right up in their face and wave their hands like that, and it seems to work. The other turtles get afraid and swim off as fast as they can. Oddly enough these same turtles have no problem sharing basking spots with each other, but once they get under water it's a different story. It must be the feeding spots they are protecting. In any case, when I had my old pond the larger aggressive turtle who claimed dominance was the one who stuck around, while the other ones disappeared, one by one.
Another reason turtles will wander is their homing tendency. Take a turtle from it's natural pond, or someone else's pond where it has been living for a few years, and place it in a new environment and it will want to try and get home. Lots of animals are like that. They get use to a certain place and know where to find food, they know where to sleep and hide, and put them in a totally new place and they lose all that and will want to return where they came from. That's why I think it's best to get turtles at as early an age as possible. before they are old enough to get comfortable in any place.
Another possible reason turtles might wander is they just don't find the place suitable. The pond might not be big or deep enough, maybe there is no place to hide under the water, maybe they see a dog always hanging out around the pond all the time, maybe all the big koi in the pond freak them out.
All kinds of reasons they could feel the urge to wander, even in large natural ponds they up and wander all the time, that's why you'll find them in so many places. It's just their natural instinct, and probably a lot more likely to happen in smaller, man made backyard ponds.