Because both of mine have done it. When DS1 was little I just thought that's what 4yo children were like, but then he was later diagnosed with ADHD so I thought oh, that's why. But now DS2 is doing it. I would say he has no other signs of ADHD, except some sensory preferences, but he's 4, so it's hard to tell.
Anyway the behaviour I'm uncertain about is this:
Going into a state where they seem "escalated" or dysregulated. What this looks like is kind of wild, staring eyes that seem to look past you, no acknowledgement or response at all from talking to them, as though they can't hear you, even when you go down to their level, place a hand on shoulder, etc - the normal ways you would help a child to engage. Or nonsense responses. And accompanied by destructive behaviour: Loud, repetitive, annoying sounds; pushing through/past other children or people as though they are objects; choosing a large toy (e.g. ride-on) and pushing that through everything with no regard; tipping containers over (for example).
Sometimes anger, often in response to being asked to do something such as tidy up, which may result in physical violence or shouting/screaming in somebody's face. Lots of laughing, which comes across as defiant (I don't know if it is defiant or anxious). Often outlandish excuses like "I can't tidy up my legs are broken" or "I'm too tired, I need you to carry me" or repeating what the adult has said "No, I want YOU to tidy up!" or repeating other adult phrases e.g. "Stop screaming, I can't understand you when you scream" (Which is fair, although we aren't screaming at him).
Often in conjunction with another child, when it happens in conjunction with another child, there is a lot of giggling and they respond to each other, but not to other children/adults outside of this little bubble.
It seems impossible - you can punish it obviously but it makes no difference as they get into the same state again the next time they hit a limit of tired/hungry/overstimulated. And honestly I'm not sure it's even really conscious behaviour. If by some miracle I can get them to calm down, they revert to their normal helpful, happy, thoughtful self. Trying to address it in the moment results in a complete whirling dervish of child kicking, screaming, flailing and more behaviour.
Frequency: I want to say most days?? DS2 doesn't do it as much as DS1 who definitely used to do it every day. DS2 it happens more at nursery than at home. I should say he is not at school yet as we live abroad but he is UK school aged. He would be younger in the year as August birthday. For DS2 it is very closely linked with him being tired and/or hungry. DS1 I can't remember as it was a long time ago (he is now a teenager).
DH is convinced that it's a sign our parenting is too lax. I'm not sure but I'm willing to try/explore anything as I can't go through 2+ years of this again. I wanted to sell DS1 to the zoo! He is now a lovely teen BTW so I'm not convinced our parenting is that shit as surely he would be a nightmare.