1. Prepositions uses

The prepositions 'behind' and 'after' share a basic meaning but the following two sentences show 'behind' and 'after' can have very different meanings.

I am after you all the way.
        This could mean you are following someone as in coming second or not going to pass just stay in the position of after.
It could also mean you are chasing someone for bad reasons to capture or hurt them.
I am behind you all the way.
       In this case you are offering support and help to someone to assist them achieve higher goals and achievements.
It could also be that you will be in that position but there is nothing sinister or bad when you use the preposition behind.

Many prepositions have similar basic meanings but when used can have very different meanings.