You liquidate rapidly if the market is way down. If you are planning on withdrawing ~4% of your stating portfolio annually to pay your expenses, the market tanking by 50% means you are now consuming 8% annually.
In the accumulation phase the value of your equities is likely to return. You never end up selling anything. But in the retirement phase you liquidate in order to pay your expenses. If you end up liquidating down too far there won't be enough future growth to cover your retirement needs.
Market crashes right after retirement are very dangerous for retirees.
That’s not to say that timing isn’t an issue — it absolutely is. It’s just not a make-or-brake issue imho.