All soil gradually becomes more acidic over time.  Under trees, this is accelerated due to debris from the tree rotting into the soil.  Unless the pH of the soil is checked every few years and corrected, the grass will struggle and moss, which prefers acidic soil, takes hold and chokes out the grass (which prefers neutral soil).  Most weeds aren’t too picky about pH so they can thrive there too.  HERE’s a link to more on checking and correcting soil pH.