There are many reasons for different backscatter of agricultural crops
- Different orbits cause different angles between the look direction of the satellite and the row orientation of the crops. Changes of a few degrees can already cause a difference of 5-10 dB (source). I would try to avoid to combine images of different orbits for agricultural studies.
- Especially row-like crops cause directional scattering which is very sensitive towards angles (source)
- Cross-polarization (VH or HV) is sensitive to ear bending of wheat crops (source). This can change quite quickly within several days.
- Stalk or root lodging of cereals, e.g. caused by high density of grains, leads to an increase of backscatter (source)
- Rainfalls lead to higher moisture content and larger contribution of surface scattering from the ground, especially over grasslands and spiked grains.