Capital flight and interest rates
It is very difficult for interest rates to cover the losses caused by a sharp devaluation. For example, if you believe that a currency is going to drop to 75% of its current value within 3 months month due to a devaluation, you need an annual interest rate of about 216% ($3.16^{\frac{1}{4}} \approx 1.33 \approx \frac{1}{0.75}$) to cover that loss.

Since interest rates of that magnitude would cause deep stress for domestic borrowers, we can reach a point where speculators believe that the central bank will not raise interest rates by enough to cover the losses (relative to a foreign currency) created by a devaluation.