It could be, but not necessarily. If you tripped and conked your head on a rock, that would also be a “blunt force injury”.
Possibly. But the story says the other guy with him also sustained injuries and “can’t remember” what happened, i.e. suffering amnesia. Sounds like evidence of an attack.
Remember both agents had head injuries, the one that died had multiple serious injuries- people at the scene said it looked like he had been beat to death. The surviving agent had serious head injury and back injuries. So a fall, even from the highway at the top of the culvert (estimated 8 feet) would not result in those kinds of injuries. Not too likely they would both trip and fall or just fall. Many of the articles only mention the agent that died but there were 2 involved.
Border Patrol from agents in the field to supervisors seem convinced they were attacked from behind by smugglers. Many BP agents have been physically attacked over the years so it is absolutely possible, to me even the most likely answer. I think politics right now has the FBI and others looking for another explanation. I could be wrong, have been wrong before.