Some plant species exhibit allelopathic tendencies, making it difficult for other plants to thrive within a certain distance of their root system and/or canopy. Same species allelopathy is rare and typically limited to certain annual plants (some veggies, for example) or plants that have evolved in high risk environments.

If it were a common 'problem', the face of the typical landscaping, which includes quite a bit of monoculture, would look very different indeed. I suggest that you consider the other factors....like proximity to a down spout, or soil contamination, or not enough water or that always aggravating " just because".

It would be helpful if we knew the kind of plant we're talking about. There might be known problems with the species, so that the loss might actually be sort of expected. Who knows, you might have had some plants that DO exhibit strong allelopathic tendencies. But if they were typical foundation-type shrubs, I doubt it.

Here is a link that might be useful: understanding allelopathy