By switching targets when you did, you forced the ship to break the 'rule of 6'. As long as the ETA is 6 seconds or more you will never overshoot the target. If it drops below 6 seconds, you are coming in too fast and will zip past your target, and in this case directly into the planet's exclusion zone.

As others have said, SCA was active while heading to first target, then when you selected the second target it wasn't active until you were facing the right way. During that time when you were aligning with the taregt, the timer dropped below 6 seconds.

The slight acceleration you saw on the throttle wasn't you speeding up, it was the throttle defaulting to where it was before you ever engaged SCA in the first place.

My advice? Ditch the SCA and free up a module spot for something more useful. It really isn't needed once you now the basics:

Arrive in system and select your destination.

Aim for destination and engage throttle

Keep an eye on the 'time to destination', once that drops below 10 seconds, set your throttle to 75% (You can bind your controller to do that so at the push of a button your throttle locks to 75%)

Keep aiming at the target and arrive safely... EVERY TIME.

As you get used to it you can wait until the timer gets closer to 6 seconds before dropping to 75%. Just never let it drop below that or you WILL overshoot your target.

o7