Sometimes you wonder what you've sent earlier, or you need again something that you already wrote earlier. But if you wrote a command, depending on how it ends, when you get back to it, it can suggest something, making the up and down arrow keys switching between these suggestions (so what they do suddenly change).
You can hit escape to close the suggestion box and continue what you were doing first, but it can quickly become annoying.

What I've noticed to do that (but I'm pretty sure it happened to me for other things as well) are target selectors and potion effects. So this willl happen, for example, for these completely valid commands :
/kill @e[type=!player]
/effect clear @p minecraft:fire_resistance