the patterns in your interferogram and the ramp in your displacement indicates that your result is not reliable at all. It is just randomly aggregated phase noise. The fringes should at least be connected over the area you want to observe.
This is most probably because you use images from 2014 and 2018, which is a way too long time for differential InSAR, because your signal is completely decorrelated.
These things are well discussed in here, you might have a look: Some explanations about concepts of fringes of interferogram and coherence
High coherence does not indicate high displacement, it simply is a measure on where your signal can be used for displacement mapping. But this only works if the area of interest has connected areas of higher coherence because otherwise unwrapping errors occur and produce a trend as seen in your result.
Is your study area covered by vegetation? This is one of the major reasons for decorrelation. I would suggest using image pairs with smaller temporal baselines but if there is much vegetation, you cannot do much about it at all.