Basically, IPA isn't a very polar molecule. When you add ketone to IPA, to further decrease its polarity, it'll form its own layer. Ditto with adding something like salt or sulfuric acid to water, you'll increase the polarity of the water, and push the IPA into a non-polar layer. Same thing happens with n-propanol as well as isopropanol. Lower alcohols like EtOH and MeOH will not tend to do this, usually the lower alcohol will stick with the aqueous layer...
And higher alcohols, like n-butanol and n-pentanol aren't even fully miscible with water.