I haven’t read about the specifics of this Tetris crash, but usually what happens with these old games is that memory is very tightly packed, imagine you have a small version of Tetris that has 3 digits XYZ where X is the speed of the game, Y is the amount of lives and Z is the level you are in, so for example if you’re in speed 5 with 8 lives on level 7 the number would be 587, if you go up one level it becomes 588, now on that example if you’re on speed 9 with 9 lives on level 9, i.e. 999, and you go up one level the number becomes 1000, but because only the 3 last matter you’re now on speed 0, with 0 lives on level 0, since speed zero means nothing moves you crashed the game.
Again, this is not exactly what happened here, but probably something similar where increasing a number overflew to the next one in memory and that caused some weird behaviour.