What are some reasons Canadians may be denied entry at the U.S. border?

The worst mass shooting in modern Canadian history began almost five years ago, shortly after a Nova Scotia man brutally assaulted his common-law wife.Lisa Banfield was kicked, punched and choked by her partner of 19 years on the night of April 18, 2020. She was left with fractured ribs and vertebrae but managed to escape. Over the next 13 hours, Gabriel Wortman fatally shot 22 people in rural Nova Scotia before two Mounties shot him dead at a gas station north of Halifax.