I think that a reason for this phenomenon is that game improvement and time management are conflicting attributes of the game.
As you seek to improve the game, more time is spent in calculating the best moves and ensuring minimal blunders. All these come at the price of a slice of the clock. More time spent on the board means fewer time for subsequent moves. We are humans, so no matter how good we are, we tend to panick when the time on our clock is very low (computers do not lose their ability even in time trouble).
Part of being able to play blitz is time management. You cannot be analysing every single move, because time is worth a lot in such time controls. Remember that a fair number of players also try to flag their opponents when they know that their opponents are in time trouble (especially in non-incremental games).
It's a trade-off between the two. Remember that there are also players who also face blitz and rapid ratings in opposite directions, but the other way - that is, their blitz ratings go up and rapid ratings go down.
Hey all, just wondering if this is normal, my rapid rating is currently skyrocketing to 1800, yet my blitz is going down the drain towards 1500, when it used to be in the 1700 range. I mean I don't mind since my primary goal is to get better in slower time controls, specifically reach 1800 rapid, but still, seems strange to me why this would be the case. Should I care at all? (Edit: Perhaps having a bullet phase killed my blitz game?)