plz read! Question in roll value of residential land

[Soran Azizi]

Hi I'm 23 y/o, recent college grad and I'm looking into starting off small with purchasing some lands in SoCal.

my question is in regards to this land's roll value history.  What could be the reason of such big dip in the net value of this property? what does this signify? is it safe to be purchasing these kind of lands given the  assessed value. 

thank you for reading this, I'm very new and would like to clarifications.