This exclamation mark menu item

The chances are determined once you open/unbox. The cases don't have pre-existing loot that you unbox.

And no, the time of the unbox does not affect your chances.

The chances have not been officially confirmed by Valve.

You cannot stop this. The most you can do is (as @DmytroTsiniavskyi said) mp_respawn_immunitytime 0 but this will completely disable those few seconds of immunity when you respawn so you can easily get spawn killed. I would not recomend this. And sadly, I could not find any plugin or clientside anything to disable this.