Well, it is Forcey. But the canonical explanation was never really there. For a long time, it was just assumed that Jedi vanish into the Force when they die, so it happens a lot in the Tales of the Jedi series.
Then Qui-Gon didn't disappear on death, nor did Maul, and nor did the hundred-plus Jedi shot to death on Geonosis. So, it went from "what happens when Jedi die" to "some kind of magical power." Which was then exploited by Mara Sue when she was killed; she delayed joining with the Force on death so that her body would be discovered and be evidence of Jacen's treachery or something... meaning Zahn isn't the only one who writes Mara as a ridiculous war goddess.
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