Answer:
To understand why Spain was threatened by La Salle, it is important to understand who La Salle was.
La Salle or better known as René-Robert Cavelier, sieur de La Salle, was a seventeenth century French explorer who led an expedition to Illinois and Mississippi River Delta. He claimed the lands of Louisiana for King Louis XIV of France. By this claim, he wanted to extend the lands belonging to the New France to the borders of New Spain. There lies the threat.
It was purely a territorial threat.
Explanation:
Till today any territorial threat that approaches another country's borders is always not welcome. In the same way, Spain felt threatened when a French citizen was trying to expand the territory of France where Spain had already laid claim to. It is noteworthy that René-Robert Cavelier was a member of the La Salle religious group before he lost his vocation and decided to explore North America.