Something happened to make the baby cry, and the cat assumes the adult is at fault. This cat is a serious baby defender! Warning: heavy metal ahead. -via Unique Daily
Any of these theories could be correct. Yes, the cat could be protecting the child, though if that were the case it would likely put itself between the adult and child - which it does not always do. More likely is that it did step on some glass and react accordingly. The point as also been made that this might not be the first time the cat's done it, though, I think that given the reaction of the mother/nanny, it likely was (to this person, at least).
Furthermore, to all of those who say you love your cats but would punish them for behavior like this - that's despicable. Chasing a cat down and beating it for doing this is not right. A cat is a domesticated creature that ultimately has little power over a human who can easily throw a towel over it, pick it up, and put it in a crate or another room. Yes, cats can bite and scratch, but usually do so because they are frightened. You as the human being can easily accidentally kill an animal in your anger, whereas the most they can do to you are some unpleasant bites or scratches. You should be ashamed for even saying such things.
Thank you for being a noble and intelligent human being.
Furthermore, to all of those who say you love your cats but would punish them for behavior like this - that's despicable. Chasing a cat down and beating it for doing this is not right. A cat is a domesticated creature that ultimately has little power over a human who can easily throw a towel over it, pick it up, and put it in a crate or another room. Yes, cats can bite and scratch, but usually do so because they are frightened. You as the human being can easily accidentally kill an animal in your anger, whereas the most they can do to you are some unpleasant bites or scratches. You should be ashamed for even saying such things.