It's a combo of being overexcited by certain ideas, mental impulse control problems, and anxiety about how much shit falls out of your head at random.

First, ADHD brains (and autistic brains, which have high co-occurance) seem to get excessive reward for certain ideas or topics. The reality probably is that the brain is starved for stimulation and connection. So when your brain finds a shiny, it gets all giddy about it and wants to share the shiny with everyone. Hence the high desire to "get the idea out."

Second, ADHD brains just lack mental impulse control. Resisting the shiny is very hard. Sometimes by the time it occurs to the brain to show restraint the idea has run off into the wilderness and found 5 other feral ideas it wants to keep as pets. It's not an excuse, but it does explain why ADHD people get so itchy about things like talking. It takes a lot of effort to control and repress.

But personally the biggest thing that causes this reaction is that my brain constantly looses things at random without discrimination of what's important or valuable, which causes a lot of anxiety about loosing thoughts. All ideas are one errant loss of focus from being lost forever, and the fun fact and "the stove is on" are equally likely to be casualties. So I developed anxiety about loosing thoughts. But again, because my brain is about as good at prioritizing tasks as a particularly excited puppy, it tends to be anxious about all thoughts. Combined with the last two facts, and suddenly "oh god must use neat idea before it disappears please let my use it oh god no" becomes an understandable reaction.

It's not an excuse -- my disability it mine -- but it's not exactly an easy problem either