It seems to be quite destabilising for autocracies. Some might consider that a a good thing though

Autocracy is destabilizing for autocracies. Looking at history, autocracies are constantly prone to being overthrown from within, and from without.

This is a feature of the autocracy being overthown, and not necessarily a feature of the system of government that replaces it.

> autocracies are constantly prone to being overthrown from within, and from without

And this is quite fundamental to the autocratic model. Coming in #2 in a democratic contest for power means you don’t get to be President. Coming in #2 in an autocratic contest for power means losing your property, liberty and likely life. The former encourages reconciliation. The latter, challenging the outcome at all costs.

To build on that, coming in at #2 in a democratic contest for power means you don't get to be President yet. A properly functioning democracy affords you or your faction a second chance usually enhanced by accepting the result. A properly functioning autocracy doesn't.

>In what way is democracy to be considered destabilizing?

I think it's interesting to think of the receiving side of this as well: how stable are democratic allies when their foreign policy can change election cycle to election cycle?

The foreign policy choices of liberal democracies tend to be far more stable, consensus-driven and cooperation-oriented actually. This is what the whole notion of a "liberal international order" ultimately boils down to. And it was hardly a failure - even the most recent and salient seeming "successes" of various more-or-less authoritarian polities are ultimately reliant on the "liberal" order working as well as it does.