What causes high blood pressure spikes?

Diet. Hbp rises due to eating a diet full of salt and water. People with hbp have difficulty excreting excess salt and water. If a person with hbp eats a diet filled with salt (hamburgers or steaks) and fluids (excessive coke or beer), then they will see a spike in their blood pressure because they can't get ride of this extra salt and fluid normally. Not taking your meds will lead BP spikes as well.
Many things can. Like stress or anything else that triggers adrenaline release for starters.