Well, your condition says "Frog is overlapping log", so yes, from the moment the two objects will be in touch, they'll be pinned.
And so it means from the moment the frog touches the edge of the log.
You should add more conditions to the "Overlapping" event to check out the current Y position of the frog in regards to the log :
Frog is overlaping Log
Frog.Y > Log.Y - 24
Frog.Y < Log.Y + 24
Those two added conditions would check a range and only pin the Frog when it is about 48 pixels from the Log.Y position.
Adjust this value accordingly to the height of your log.
Also you can add a condition Set Frog.Y to Log.Y in order to have the frog in the middle of the log (assuming your origin point is in the middle) before the pin action.