Tim Minchin has a great line about this in one of his songs[1]
'Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved'
The reason it violates Occam's razor is because it is the strongest possible axiom you can include in a system. Why does anything happen? Because God. That's not a very interesting theory in that it can predict anything without offering any reasons or insights.
Not to mention, we have no direct observational evidence of such an entity.
God is not a hypothesis, but more of an theory in the analytical rather than scientific sense. That is, God is a conceptual framework by through which other information is interpreted, not a source of testable predictions.