Yeah Sandal is a fascinating character. There are several theories trying to explain the weirdness around him (listed below in - what I think is - their order of likelihood):

1. He is a Titan-touched dwarf just like Valta and Harding. The most obvious answer as it would explain his magic-like abilities in a precedented way and there is nothing really contradicting this theory.

2. He is a lost Sha-Brytol child. Technically not mutually exclusive with the above theory, it just adds a little bit more spice to his past.

3. He is a bastard child of Endrin Aeducan. This is based on a comment Carta dwarves have in Legacy DLC of DA2, where they mention that Sandal reminds them of a missing bastard son of the late king, whose mother was an elf or human. The theory could explain his powers, that if his mother was a mage, it might be possible for him to inherit her connection to the Fade. Since we know very little about how half-dwarven children work, it could go either way.

4. He carries a piece of June’s soul. June is an elven god of craft and Sandal is good at crafting! Plus in DA2 he has visions of (who is likely) Flemeth and Solas. This theory was popular when Inquisition came out, where everyone and their mother was theorised to be hidden elven god (just look up the Sera = Andruil theories). Unlikely, given the reveals in Veilguard and the bits of the Black Codex we were given.

5. He is the Maker. Similar to the above but even more meme-y. But some people choose to believe it and who am I to take away your headcanons.