But why can't it be a spontaneous mutation word?

It is. It' a typo. Funny enough, Google Books returned a lot of ghits for hascent, but upon inspection they all turned out to be misreadings for nascent.

Must it be traced back to Latin?

Nope, but some of us here enjoy the chase immensely. Others prefer just speculatin', which is okay, too.

Ceci n'est pas un seing.