Changes in the real estate market are the likely reason for your value change. Since the county assessor is required by law to assess all taxable property at market value, when the market changes significantly, assessed values follow suit.
Assessed value changes may also be due to physical changes to your property that affected its market value. Additions, renovations, demolition of existing structures, or physical deterioration are common examples. For agricultural land, changes may have resulted from a reassessment of agricultural land based on new soil classifications required by legislative change to Neb. Rev. Stat. §77-1363.