Is this behaviour passive aggressive

[whatwouldyoudoifisangoutofkey]

A very old friend has a habit of becoming almost silent , using very minimum of words.
Adopts a very cold attitude, almost no emotion.
She does this as far as I can work out if she feels that someone has been impolite to her, disrespected her.
Is this passive aggressive.

Yes. Classic passive aggressive behaviour borrows behaviours from aggressive and passive to make itself difficult to challenge. Here you're seeing the clear 'I'm not happy, it's your fault' of aggressive, coupled up with 'I'm not saying there's a problem because I don't want to risk being challenged' of passive.

The only way past it is assertive.

"Is there something wrong? If you tell me, I can understand/help/change something. If you don't tell me then I have no way of knowing".

If they choose not to tell you, leave them to it. Don't feed the drama/sulking, just keep offering to try and help sort it out if they explain/communicate. Otherwise they can keep it as their problem.

Thank you . Yes ,I was tempted to ask what was wrong.
It's powerful behaviour, quite intimidating.