As I explain my books, this is called rippling -- and in most cases, it's how you would like your timeline to behave. If you insert or remove a clip from your movie, you want the rest of the movie to shift left to fill in the gap or shift right to allow you to insert it.
To override rippling, hold down the Ctrl key as you add your new clips.
Unfortunately, Track Locking is not a feature of PrE, though I have filed several Feature Requests. PrPro has that capability, since at least PrPro 1.0.
However, as Steve points out, using the Ctrl modifier key, changes the behavior, and turns off the default Ripple Edit function. Because of the inability to Lock a Track (Audio, or Video), I recommend that one not add an Audio Track (other than the camera Audio that accompanies the Video), until the editing is done. It can become a bit "messy." Now, if one is editing a "music video," that is not the ideal workflow, and Track Locking would be very, very useful.
Good luck, and if you agree that Track Locking is useful, I invite you to also file a Feature Request.
I've had that happen before, as well.
What I do is click only on the video, so the specific clip is hilighted, unlike the rest of the timeline. Then I split and it only changes the video. I generally use the scissors icon to the bottom right of the monitor, next to the text and screen capture icons, if that helps.