Here's are some hypothetical examples of why this "sit down at protests" nonsense is so bad.

Delaying March in Front of Aggressors

We're leading a march, and there's a bunch of aggressive counterprotesters on the side of the street. The protesters are reacting, and the situation is getting tense. It's a family friendly march with vulnerable people present; so, letting it turn into a brawl or letting it turn into a police response situation are not options.

In such situations, frequently your best option would be to keep separation from the agitators with a wall of marshals and then move the march away from the agitators.

But now, in 2025, we're going to have a bunch of protesters who read this "sit down if things get aggressive" shit, and they are going to sit down.

The sitting protesters will block the people around them from moving and will create confusion. So now, we'll have to allocate marshals - during a conflict - to getting the sitting protesters off their asses or out of the way.  But probably, some of their stupid online instructions imply "sit even if marshals tell you otherwise"; so, they won't move. Maybe they'll tell other protesters to sit down.

At best, they delay our movement and increase the chance of violence from the agitators. At worse, they either stall the march completely, greatly increasing the risks of escalation or police intervention; or we get the march away from them, but now we've left a bunch of people sitting in the street in front of potentially dangerous counterprotesters.

Can't Show Strength

Sometimes there are aggressors harassing a protest, and leaders may decide the agitators are bullies and that a show of force from militant but organized protesters chanting over their threats may demoralize the foes. But now, when showing force, we'll have people sitting down - not only weakening the unified front presented, but blocking and confusing people who would otherwise help.

Hobbling Escape

Worst case at protests, there are attacks - physical, vehicular, cops on horses, or tear gas. In all these cases, having people immobile on the ground blocker other people's escape is a terrible idea. Tear gas sinks and is strongest near the ground. People sitting are more easily trampled by horses.

The False Premises

This shit is based on the false premise that being non-violent leads fascists, authoritarians, cops, and right-wing agitators from doing violence. That's obviously not true. People are being murdered for being liberals or being protesters.

There is also some sort of attempt to call back to Ghandi or MLK. But here's the thing: their displays of non-violence in the face of state violence was planned. People trained and agreed to remain peaceful even as they were beaten with clubs. It earned sympathy for the protesters and shame for the the authorities. But you can't achieve that by just asking random protesters to sit down. It takes preparation and commitment.

A common argument is that "we need to be peaceful so right wing media won't spin it against us". Where the fuck have you been the last decade?! They spin everything! Truth doesn't matter! Decorum doesn't matter. This is respectability politics at it's worst, and respectability politics in the face of fascists is a hair away from collaboration.

Divisive 

This nonsense is also a blatant St Paul Principles violation. Many of the posts call out groups like anarchists specifically, implying their protest tactics are invalid. Such statements create conflict within the movement, and they serve right-wing narratives against protesters.

Alternatives

Ideally, you should go to protests with experienced leaders and marshals. If they know what they are doing, following their instructions is one of your best bets to stay safe.

Otherwise, retreat slowly while being mindful of your surroundings. Don't sit. Don't panic. Don't stampede.

If you are committed to this stupid idea of sitting at a protest, move out of the street and away from the body of the protest. By doing so, you don't impede the movement of others, and you reduce your own chance of being trampled.

Conclusion

This shit increases the risks of conflict and violence because it wrestles control of a march from experienced leads and marshals to have people follow some arbitrary and dumb online instructions.

STOP SHARING THIS SHIT. Sharing it is doing harm. You should be suspicious of people promoting this - many are either naive and dangerously arrogant, or they are some sort of an op or a bad actor.