Where are you heading (to)? Why can TO be omitted?

My Japanese friend asked me why "to" can be omitted in "Where are you heading?"

He asked me if it's because "where" is an adverb but I don't think that's the reason ...

Can someone please help me explain the reason?

The Japanese often use "to" ni to describe direction.

Tokyo ni (towards / in the direction of Tokyo)

Your example question

Where are you heading?

is a shortended form of

Where are you heading (off) to?
To where are you heading (off)?

A heading is a navagational term meaning in the direction so the "to" can be implied.

If you are driving, you might say

We are heading east on I-95.
We are heading towards New York City.

The British will use the term

When are you heading off? *(BrE)
When are you going / leaving? (AmE)