I think the whole point was for the viewer to use their imagination about what is happening.
The deer stuff did seem weird but if you are trying to convey a sense of chaos and uncertainty, they do add to that.
Discussion
Oh sure, I definitely agree and could get behind the idea of keeping the cause unknown. After all, if something like a nationwide EMP happened and all communications were knocked out, we really would be in the dark, not knowing. So I understand. But as someone who grew up on a farm, lived in the woods and have been around animals and deer my whole life.. that was the part the ruined it for me personally. All the other chaos was mechanical and technological, and whats this deer thing? Supernatural now? It just didn’t fit well IMO