Doorway tiles cracked; causes and suggestions

Any thoughts about the likely causes of this crack in case it affects repair options?
I have flooded the crack with superglue (the glass repair type) for now to hopefully prevent sharp glaze shards working loose. One may be evident in the photo just to the left of the centre grout line.
Can be covered with the doormat. Hopefully that doesn't hide something more serious. No other cracks evident.
Tiling was done ~12 years ago. Door was replaced ~3 years ago.

Cavity under maybe.
Seen it many times where the cavity was all around house including doorways. Cavity covered in felt or something and concrete screed over. Should not have a cavity at doorways but sometimes you find them.
I've filled all mine in. Will explain crack