### The spell needs a save DC; the DC of the Sorcerer who triggers it is used  

In the case you offer, fireball, the magical effects require that anyone within the fireball's radius to make a saving throw.    

What is a saving throw made against?  The spell's DC.   

> **Spellcasting Ability**  
Bards, paladins, sorcerers, and warlocks use Charisma as their spellcasting ability, which helps determine the saving throw DCs of spells they cast. (Basic Rules p. 62)   
> **Saving Throw**  
> The Difficulty Class for a saving throw is determined **by the effect that causes it**.  For example, the DC for a saving throw allowed by a spell is determined by the caster’s spellcasting ability and proficiency bonus. (Basic Rules p. 62)

How do you arrive at the DC for a spell that comes out of a wild magic surge?  Use the DC of whomever triggered the surge.  Wild Magic Surge rules text (PHB p. 103) does not offer a separate DC rubric for that particular effect, so it is tied to the sorcerer who is *the cause of that magical effect.*   

Is that stated anywhere in the Rules?  Not as far as I can find.  

Does it make sense?  Yes.   

Why?  D&D 5e design principles were to make the game simpler, not more complex.  If you accept that "you cast a spell" rules text as a pointer to how the DC for a spell effect from a Wild Magic surge is determined (so that the DM does not have one more thing to rule on or figure out) it meets the simplicity standard.  

### Try the common sense test   

Should a wild magic surge from a 19th level sorcerer be more potent than from a 1st level sorcerer?  (Difference in DC being +6 to +2 based on proficiency alone) and thus harder to save against?  

Yes, common sense would suggest that this is the case.  
(If I can find a Crawford tweet on this, I'll add it).