its possible the escort was just performing a standard zig zag pattern infront of the convoy, it hadnt detected you, and its zig zag just coincidentally at the time meant he was headed straight for you.
I've had this happen to me before aswell when approaching to intercept a convoy. It is very un-nerving. But if you'd done it right, and got in there silent - just watch and see what he does, more than likely he will turn off at some point and start heading across the front of the convoy again at the opposite angle.
There are 2 other things that negate the above however =
a) he hadnt heard but was headed for you and has NOW heard you, or
b) he did hear you on your approach
This is what makes intercepting convoys so fun in GWX - it is like playing with fire hehe