The VoiceOver keyboard shortcut used to activate context menus typically does not work on Dock items.
- Use VO+D to move VoiceOver focus to the Dock.
- Use VO+ the left or right arrow key to move VoiceOver focus to the desired Dock item.
- Use the keyboard shortcut VO+Shift+M.
You would expect that this would activate the context menu for the selected Dock item.
On most occasions, what actually happens is that VoiceOver focus moves to the front open application.
On perhaps 1 in 10 occasions the context menu will activate as expected, but there appears to be no pattern to explain why it does work on these occasions.