Learn Polish: Czy ta sytuacja jest poza twoją kontrolą? - Is this situation outside your control?

The preposition poza is followed by the instrumental case when it describes a static state or location, meaning outside of or beyond. Since the situation simply is (jest) outside your control, we use the instrumental.

Does poza always trigger the instrumental case?

No, it depends on whether there is motion involved. For a static position like in this sentence, you use the instrumental. If you are talking about movement directed to the outside, poza takes the accusative case instead.

Is poza kontrolą the standard way to say out of control in Polish?

Yes, it is a very common set phrase. It translates exactly to the English phrase out of control or beyond control, and you can use it for situations, vehicles, or even emotions.

Could I use bez twojej kontroli here to mean without your control?

You could, but the meaning shifts slightly. Bez kontroli sounds more like something is left unmonitored or unsupervised. Poza kontrolą emphasizes that the situation is actively beyond your power or ability to manage it.