Tempered glass installation fail - GIF

Tempered glass installation fail.

<video controls autoplay loop style="max-width: 100%;">
<source src="https://diqn32j8nouaz.cloudfront.net/tooltalk_17482066153751.mp4" type="video/mp4">
Your browser does not support the video tag.
</video>


Previously:

High-rise glass lift fail - GIF
How glass sheets used to be made - GIF
How to cut a glass tabletop - GIF
Mouth-blown window glass manufacturing - video
Sealing insulated glass - GIF
Snapping a long piece of glass - GIF

what did he do wrong? it looks as though maybe it clicked against the u channel on the floor - maybe it initiated a small crack?