We have to consider that a density-independent factor is a variable that does not depend on how big or how small a population is for it to occur.
In these cases, we could say that the breakout of diseases that decreases the size of the frog population is a density-dependent factor since the disease spread probably for the high amount of individuals of that population's species.
The competition for food we can see in the second case also is a density-dependent factor, since the high number of those deer caused the food resource to be scarse.
Having this clear, we could say that a hurricane that severely disrupts a salt marsh in an estuary is a density-independent factor since this meteorological process would occur no matter if a species population is full of individuals or not.