Weird behaviour when I click play

I've just started learning Unity and C# (I'm nowhere near to be a programmer). I've followed the first 2 videos on Brackey's channel to learn some tools and then mess around until I get somewhere.

I've tried to make a pyramid of "cans" (cylinders) to make an animation of a ball hitting it, then with the rigidbody built-in physics see them fall. When I click play, the cylinders don't stay in place but fall down like the structure was really unstable. Also the cilinders edges go through the other cylinders. I've tried the same structure with boxes but these stay in place. Then I removed the rest of the cylinders and left only one in the (0, 1.5, 20) position. 1.5 in the Y-axis is the position where cylinder is touching the floor but not going through it nor floating

Now here's the weird thing: when I click play, the X-axis value randomly changes to a negative number between -9 and -3 but the object doesn't change position in the editor nor in the game view. Any one has any idea of what's going on?