The drastic changes in all aspects of life is the usual answer. It’s not just that you are being uprooted from your bedroom, familiar neighborhood, and local spots, it's the fact that this happens while your entire life is in boxes, so you cannot find solitude in your new place until you properly unpack.
You also might not speak the language and have cultural barriers. The first couple weeks can feel overwhelming, but the more you put yourself to the task of unpacking and making the new home feel cozy, the faster you will feel at home!