Could be any number of reasons. Are the plants that are dying ones that you hve planted or where they there before you moved in? If the former most likely is that what you have planted is not compatible with the conditions in yr garden (sun/shade, wet/free draining soil, alkaline/acid etc) in which case it's worth investigating in some garden books to help you choose the right plant for each location.
If they are plants that were there before you moved in then it must be something you have done or not done (?) e.g. maybe the previous owners watered regularly and 'cos you did not in the recent warm spell they are suffering!