Why did my car start rolling on its own when changing the spark plugs?

I got a ford 1999 taurus SE, when I was changing the spark plugs and wireset in the back part of the engine it, I felt it start rolling back. I had it in park, I quickly ran to the drivers side and put the E brake on but I still don't understand why it did that.

It drove fine after that and so did the parking brake but wtf?

You didn't have it all the way in park. The spark plugs will only cause a manual transmission car to roll because there won't be any compression with the spark plugs out. The automatic has a parking pawl that locks the tranny so the vehicle can't move.

The compression of the cylinders stopped the vehicle moving in park.
Once you removed the plugs, the engine compression disappeared and the vehicle moved.

When you take out the spark plugs you relieve the compression and the engine offers no resistance.

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