How are perfectly rectangular icebergs formed?

Here is a transcript of the video.

Narrator: Does this image appear to be photoshopped, fake or even created by extraterrestrials? You would not be alone in thinking that. But you would be wrong. It's actually a natural phenomenon. What you see is a tabular iceberg. Tabular icebergs form a plateau with a flat top. Unlike non-tabular icebergs, which usually have a rounded top. This iceberg was photographed in October by NASA scientist Jeremy Harbeck. Most icebergs look like this at first, although this one is exceptionally rectangular. So, how are these perfect icebergs formed? All icebergs form when a piece or layer of ice breaks off from a large ice or glacier. And because of the innate crystalline structure of ice, it tends to break along straight lines.

Dr. Jan Lieser: The icebergs we usually see are altered. They are beaten by the winds and the waves, they lose pieces here and they lose some. This is a recent sculpture event.

Narrator: This iceberg is detached from the Larsen C ice shelf in Antarctica. Over time, the wind, waves and warmer water will degrade the iceberg and break it, eventually turning it into a rounded shape that you probably know better.

Dr. Jan Lieser: These regular forms, which will not stay long. If we look for this iceberg today, which is only a few weeks after its discovery, it will not look like that at all.

Narrator: When this tabular iceberg floats and changes shape, it will not look like the iconic iceberg of the photo. But it will continue, thanks to social media.