Overdosing too much nutrients/minerals into the soil can block the
absorption rate of other nutriets/minerals from the soil. There is an
optimal balance between the ppm of the various & minerals that will
result in the maximum uptake efficiency of the plant.

Also, too high of a concentration can lead to the soil containing
too much dissolved salts, and water then has to come out of the plant
in order to restore the osmotic balance--causing plant burn and
dehydration.