"who knows what they're protesting about? Do they even know themselves?" is one of the oldest lines in the book for dismissing a protest.
However I suspect that if the actual point of the protest is pointed out the goalposts will shift first to "but it's not really a relevant form of protest" and ultimately to "is police brutality really a 'national' issue?" and if that one was addressed it would become "is police brutality really a race issue?"
This is the most common pattern. Someone's already decided which "side" of an issue they're on and will always and only present arguments that back up their "side". Someone who's actually open to debate will consider points both for and against something, altering their position as new evidence is presented, while someone who already made their mind up will consider all actual new evidence to be irrelevant and will just present an endless number of unrelated arguments/points that are thinly veiled rationalizations for their original opinion/bias, as each previous one is debunked.
Comparing kneeling during the anthem to "beating your wife" would seem to be one such non-sequiter argument that only serves to justify the origional biased opinion. It certainly has no logical connection whatsoever to any of the original complaints, so it fits the pattern "come up with endless random, unrelated and illogical reasons to justify not liking something".
I mean, if someone just constantly comes up with random reasons to dislike something, then either they are the one who doesn't quite know why they don't like it, or more likely they have some other, unspoken reason for not liking it which they know is not socially acceptable. For example in this case it could be "fuck those unpatriotric non-flag worshippers" or something, which the guy realizes would not fly as a reason if stated openly.