Improper loading of the big toe/hallux ?

The callus pattern indicates were friction or pressure loading is present. When the loading is too fast or aggressive, we get a blister, but when the loads a low and over time, a callus develops. It can be from rubbing up against a shoe but it can also be from loading responses through the skin.

In this case, we see the callus under the proximal hallux and slightly medially. This can indicate that the short flexor of the hallux (FHB) may be more dominant. And we see clues here, the tip of the hallux is curved up, though this is not a great photo to represent this.
When the short flexor is more dominant, the long extensor is typically more dominant, as we see here by the big toe curling up.
When these are more dominant, the long flexor and short extensor are subservient. This presents us with some tendency toward a hammer toe response, and maybe a true hammer toe over time.
Callus patterns are clues, not answers, but they are breadcrumbs as to how your client is loading, where they are loading, how aggressive the loading is and the motor patterns they could be deploying.
Look for them, and let your examination, confirm or deny your suspicions.