A bus is coming here. This sentence is correct, but why there is a bus here is not correct.

The first sentence is an ordinary sentence. A bus is coming this way.

The second sentence here is placed at the beginning of the sentence.

When adverbs indicating position are placed at the beginning of a sentence, if the subject is a noun, they should all be inverted, such as:

Here's your ticket. According to this principle, the expression of the bus coming should be like this:

Here comes the bus.

However, when adverbs indicating position are placed at the beginning of a sentence, if the subject is a pronoun, they will not be inverted, such as:

1. Here it is!

Here he comes.