Als ich aus dem Bus steige, winkt mir meine Nichte vom Fenster aus.

Because als introduces a subordinate clause. In German subordinate clauses, the finite verb usually goes to the end.

• Main clause: Ich steige aus dem Bus.
• Subordinate clause: als ich aus dem Bus steige

So the word order changes because of als.

German normally separates a subordinate clause from the main clause with a comma.

So here:

• Als ich aus dem Bus steige, = subordinate clause
• winkt mir meine Nichte vom Fenster aus. = main clause

That comma is required in standard German.

German main clauses follow the verb-second rule. The finite verb must come in the second position.

Here, the entire subordinate clause Als ich aus dem Bus steige takes the first position in the sentence. That means the finite verb of the main clause has to come next:

• Als ich aus dem Bus steige, winkt ...

So the subject meine Nichte comes after the verb.

Because winken takes the dative, not the accusative, for the person you wave to.

• jemandem winken = to wave to someone

So:

• mir = to me
• not mich

Examples:

• Sie winkt mir. = She waves to me.
• Ich winke ihm. = I wave to him.

That word order is very natural in German. Short unstressed pronouns like mir often come before a full noun phrase like meine Nichte.

So:

• winkt mir meine Nichte sounds very natural

You could also say:

• winkt meine Nichte mir

but that usually sounds more marked or emphatic.

Aus takes the dative case, so dem Bus is dative singular.

• der Bus = nominative
• dem Bus = dative

German uses aus for movement out of something. English often says off the bus, but German says out of the bus/vehicle:

• aus dem Bus
• aus dem Auto
• aus dem Zug

So this is normal German usage.

Yes. German can express this idea in more than one way.

You may hear:

• aus dem Bus steigen
• aus dem Bus aussteigen

Both can mean to get off / get out of the bus.

The version in your sentence uses the simple verb steigen together with aus dem Bus. A lot of speakers would also naturally say aussteige here.

Von ... aus is a common expression meaning from a place or from the position of a place.

So vom Fenster aus means something like:

• from the window
• from her place at the window
• standing/sitting at the window

The final aus belongs to the expression von ... aus.

Because the full expression is von ... aus. It works as a unit.

Examples:

• von hier aus = from here
• vom Bahnhof aus = from the station
• vom Fenster aus = from the window

So the aus is not random; it is part of that fixed pattern.

Vom is just the normal contraction of von dem.

• von dem Fenster → vom Fenster

This contraction is standard German, not just casual speech.

This is a very common learner question.

In general:

• als is used for a single one-time event
• wenn is used for repeated events, general situations, or if/when

So als suggests one specific scene. In your sentence, the present tense gives it a kind of vivid narrative feel, as if the speaker is describing the scene as it happens.

A neutral general statement would more likely use wenn:

• Wenn ich aus dem Bus steige, winkt mir meine Nichte.
  = Whenever/when I get off the bus, my niece waves to me.

But with als, the sentence sounds like one particular moment being narrated.

It is nominative, because meine Nichte is the subject of winkt.

Even though it comes after the verb, it is still the subject:

• winkt agrees with meine Nichte

For feminine singular nouns, the nominative and accusative forms often look the same, so word order alone does not always tell you the case. Here the verb and sentence structure show that meine Nichte is the subject.