Why my results become a rectangle

hi,could you tell me why my results (Fig. a)become a rectangle when i estimate the velocity of glacier(Fig.b) by SNAP,thank you very much.
(Fig. a)


Please share some more information on your processing steps (what was done between a amd b) and what you mean by rectangle.

thanks for your reply. i done this refer to the webinar (RUS Webinar: Glacier Velocity with Sentinel-1 - CRYO01 - YouTube step by step, and all the parameters are from this training,and the raw picture like Fig.b. but the result like Fig.a. I wonder if a part is cropped during the calculation, but two pictures are inconsistent direction.
By the way.i have read some papers that some people estimated velocity of glacier by SLC products, which has more information about glacier,however, those products is big and complex that need to do lots of preprocessing , but other people estimated it by GRD products,which saves a lot of processing that could use directly. my job focus on Antarctic in the future, which products are better for me to estimate glacier velocity in Antarctic. could you give me some advises ,thanks a lot