Theoretically, it takes an awful lot of prolonged physical labor to dig a hole big enough to bury an adult. If the ground is soft and wet, thats worse. The water will makes each shovelful of earth heavier than if it was dry.
He was likely already exhusted by the strangulation, now, he’s in a panicked hurry to get the job done before somebody get’s nosy. His clothes are all dirty now.
All it would take is for someone to quietly walk up behind him and calmly ask:”Hi Bill, are you starting a garden today?” to shock him into a heart attack.
He dug the hole the day before.