He wasn't really cordial to me after a while - I don't know if it was my response to the branching dialogue that did it but he suddenly turned menacing which only encouraged the thrashing I gave him
Either way, Susano's summoning seemed too convenient to me to be merely 'accidental' - I kept expecting an Ascian to be seen lurking in the shadows (alas there was none).
We do know that Primals
can be sealed within objects (Bahamut was after all sealed within Dalamud, Odin within Urth's Font, and the Warring Triad within that ancient Allagan tech in Azys Lla, to say nothing of Primals being sealed under Silvertear Lake which were released when the
Agrius crashed into it and exploded), so who is to say that Susano was not some ancient Primal from however many years ago that was sealed within those three treasures of the Kojin (remember also that the Kojin believed that 'kami' inhabited those objects, sounded a lot like aether to me), which reuniting the items together... broke the seal and released him.
Either way, there is definitely precedent for his unexpected summoning.