That crazy cat just had to push his luck 
Little quicker than the dog... Although I dont think the dog seen the attack coming.. Crazy cat 
I highly doubt the dog was blind-sided. As a matter of fact, I'd bet that if we could see the dog in the initial part of the clip, its behavior actually led to the attack (with the help of the dogs owner).
Watch the the doe. She gives all sorts of indications of what's going on off-screen. Right in the beginning of the clip, she's concerned about the cat, and comes in ready to defend her fawn from it. She quickly realizes it isn't a threat at all, but rather just an annoyance. She calms down; until she see's the dog at about 50 seconds into the clip.
Her tension escalates until she's finally had enough at about 1:10. Before that, the fawn uses its defensive behavior (maybe alerted by the does behavior?).
My guess is that first of all, the dog knew the deer were there.
Having dogs of my own, I cannot imagine it taking them more than an instant to detect the deer, especially since the deer were out in the open. Considering that when we first see the dog, it's showing what appears to me to be submissive behavior and is heading towards the person who called it's name (who's calling its name from the other side of the street, and further away from the deer) I'm betting the dog was out of its yard, and was headed towards the deer in the time between 50 seconds and 1:10 in the clip. It may not have been/probably wasn't "attacking" the deer, but I bet it was interested in (and approaching) the deer.
It wasn't just standing there, and it didn't just appear. It's very unlikely that the doe wouldn't have detected the dog, so it (the dog) probably isn't on the scene at all initially. It comes on the scene sometime
after about 40 seconds into the clip.
The does behavior shows this. She probably wouldn't have even approached the fawn initially, had the dog been there. The fawns best chance at survival is to not be detected. It has no scent at this age, and is programmed to lie down flat and be still. If the doe approaches the fawn with a threat nearby, she defeats those defenses, and leads to her offspring's death. The cat throws a wild-card into the fray, though. She may have been "forced" to approach her fawn, thinking she'd need to defend it from the cat...
If it (the dog) was aware of and interested in the deer, its body language would have been showing that. Tied with approaching, that's bad news from the perspective of the deer. Working-breed dogs like that would be very likely to be approaching in a manner that looks predatory, like stalking, too. Low to the ground, using fairly short, repeated, advances.
At 1:11, the lady calls the dog. The deer isn't to the dog yet. This is when the lady (who'd probably been watching the whole thing...) recognizes that the deer is getting serious about defending her fawn, and decides maybe she should call her dog back. At this point, the deer is threatening the dog by advancing, but still hasn't really "committed" to the attack yet. She could easily still be "bluffing". AT 1:14, it looks like something happened (that we can't see) that either convinces the doe that attack is necessary (which I doubt, since the threat level hasn't escalated), or that her attack has a high probability of success (she be able to defend the fawn, without getting injured herself). Were I placing bets, I'd say that this is when the dog turned away from the deer (towards the lady calling it). Turning its back on the doe was bad for the dog. Had it stayed facing the dog, the doe would have probably stopped and postured a bit. Turning away from the doe opened the door for attack. At the same time, recognizing the threat of the approaching deer as well as the tone of the lady's voice, the dog likely exhibited submissive body language. Now it's in trouble.
The doe had to take advantage of that, and right away; her fawns life was hanging in the balance. The situation could quickly change, and she'd have missed her chance. Failing to defend the fawn will generally mean a dead fawn, in the "real world". The doe has no concept of "right and wrong". And fat, over-fed, lazy, "soft" predators aren't the norm, in her world. Normally, this is a deadly, serious game, that gets played over and over and over and over. And in the "real world", her fawn wouldn't likely survive this. Desperate times call for desperate measures...
