you can only retrieve reliable information from InSAR when the waves of both signals are “in-phase”, that means if they were received from the same scattereing mechanisms on the ground. If a surface changes between the first and second image (e.g. movement of a tree canopy, growth of crops), this is no longer the case and your signal is out-of-phase, resulting in a decrease of coherence. That is why we look at the coherence band as an indicator for the phase quality. It sould be bigger than 0.4 for large parts of the area you want to analyze, otherwise the interferogram is likely to contain noise which does not represent the actual surface condititions. As a result, the unwrapping will give you a quite random or heavily distorted result (as the ramp in your image).
These might also help: Some explanations about concepts of fringes of interferogram and coherence
If you are new to InSAR, I strongly recommend the documents compiled by ESA:
Also, the tutorials cover many practical aspects and considerations: