I think like guys mentioned in here its a lot of different factors, from being anxious, to feeling out your opponent, gathering data, or just needing to wake up.
I suffer from this as well. When boxing for some reason I cant get my head completely in the game until I get cracked good a couple of times, but when that happens its like my brain shifts into next gear and I suddenly get that crazy tunnel vision and then I start fighting to my actual capabilities.
It sucks. I always thought it was maybe the nerves, but it also always happens during sparring, and I have zero nerves for sparring. Its like I’m stuck in this mental mode of still sitting on the couch chilling, and when I get a good whack I start fully realizing I’m not chilling at home anymore.