My first guess would be that to be a Muslim in a Christian majority country is fundamentally different from being a Christian in a Christian majority country. The latter can easily be a result of social inertia, whereas the former is more likely to be a strongly-held conviction. Which is why a general secularization of society would have a strong effect on the majority and negligible effects on minorities.
In other words, a combination of your 1 and 3.
Also, I suppose there might be class issues. Muslims in England tend to be poorer, and non-religiousness is correlated with higher education.