Why would a justu activate itself?
The caster (Obito) needs to activate it, after that the phasing works subconsciusly
If kamui activates by it's own then Obito wouldn't even be able to interact with anything in the first place, and has to wait 5 min phashing just to touch something for an interval, and then wait another 5 min
Fourth Databook, along with Obito's flasback and DMS Kakashi. By subconsciously, I think it means that intangibility is the default state of the eye and the user has to make a conscious effort to become tangible; in the absence of that effort, it switches back to intangibility. The other aspect is that the user doesn't need to focus as to which part gets warped to the Timespace.