during the cooking process.

I know the open the door problem but if the door hasn't been opened what are the variables that cause a cake to sink during cooking. My confusion is because it doesn't do it every time which means I'm not consistent about something but not sure what.

It is a devils food cake with bicarb, self raising flour, sugar, cocoa, milk, vinegar, melted butter, eggs, vanilla and baked at 170 (or less - are the words).

Would using an electric mixer vs spatula/spoon be a cause???
Oven temp???

Any advise appreciated