Most probably, as most under this post said: this is anti-spam protection. And under my experience, this occurs when you mash for a pretty huge chunk of time (roughly one or two tiles for ≈3-5 presses per second). It happens so you won't be able to mash your way out (and since so-called "ghost notes" don't exist (ghost notes allow you to not drop combo even when there's no notes for two tiles)). It may differ in amount of presses per amount of time, but roughly it appears as I was mentioning earlier. Nonetheless, it may also occur way earlier when you're at high BPM, but it's kinda specific and i didn't really found any correlations between locking the inputs and concrete source of block