Did you disconnect the battery from the systemboard before spraying the IPA, you didn't say? If not there's the problem.
Even if the phone is off, there's always power somewhere on the systemboard. The power button is not a power isolating button. Its function is to signal the intentions of the user i.e. turn on, off, sleep etc. Think of the phone as being in an extremely low power state when off and not that it is totally disconnected from any power
Regarding using IPA, people should always read the label on the product. It states (in most cases) to use in well ventilated areas and to seek medical advice if swallowed. The information is all there but it is very hard to protect people from themselves.
BTW I think @aactech is not quite correct regarding IPA 99%+. It will conduct electricity when still in a liquid state albeit not so readily. The only liquid that is guaranteed not to conduct is distilled water.
The beauty of IPA99%+ it is that it evaporates quickly and leaves no residues, conductive or otherwise once it has fully evaporated unlike "rubbing alcohol" which some say is OK to use, as this is usually <70% IPA and can contain additives which do leave conductive residues.