Top victim-blaming and assault-excsing there!
If he was protective of his dog, he'd have it under close control, like the CRT code says. The old bargemen wouldn't have hesitated to hook a loose dog like that into the water before it troubled their horse, so the owner should be thankful he only has to watch for cyclists these days!
How is that dog behaving out of control or even not in control? It was better behaved than the man!! There is no rule that says a lead is the only way to control a dog and indeed putting a lead on a dog can result in a totally different behaviour from the dog that is possibly worse than the same dog without a lead.
For example, a well trained dog that is used to being off the lead on such a towpath and has experience of other users on here might have a tendency to check for cues from the owner as to where to be. Or they might just stand still and wait for the cyclist to pass. Our dog always used to see a cyclist and freeze then as it sees which way the rider is going it then moves past the cyclist giving as much space as possible and to get past the cyclist quickly. She does that with other dogs too. IF we stopper her when that same cyclist approached and put a lead on her then she would not be relaxed but on a kind of higher alert. She picks that up from the very action of putting a lead on when normally I would not need to or actually do.
So IMHO that dog is under control and it was not the issue here. IMHO the man was and to some extent the decisions made by the cyclist was too. That does not deserve dunking but similarly how do we know that the dunking was really due to a push and not a simple imbalance on the cyclist's part. It is easy to lose balance astride a bike when you are wheeling it along that way.
Imagine if there was a following cyclist filming and caught it as not a push but a stumble. Totally theoretical but think about it that original video is not conclusive of anything other than cyclist met a couple with a dog, some words were exchanged and the woman went into the water. Without a witness or other video there is not a case to answer and to go all defensive over the poor woman cyclist and jumping to conclusions is not getting us anywhere.
Perhaps look to what we can see and the video proves. What has each party actually and evidentially done that was right and less right. I do not think the woman has done anything wrong I just think she has not done it the best way. As I said earlier she could have stopped earlier to let them walk to her and past her. She could have gone more to the hedge side so the water was less likely to be the end result for her, no matter how that might have happend. She could have got off her bike as soon as she realised the guy was reacting. Although an earlier stop might have meant he did not react, we do not know for certain as people have automatically jumped to the conclusion that he is a nutter who pushed a poor cyclist into the water. There is what is right and there is what it better or best. Conflict is not necessarily the result of a cyclist / pedestrian interaction on a canal towpath. It is possible this might not have happened if things were done differently by the man and the woman cyclist.