I have no proof, only whispers
Is it fastest growing in Europe because the native pop is becoming irreligious and Muslims are reproducing at higher rates/immigrating?That's true, most of our numbers are growing because of high birth rates, mashallah. However, Islam does attract a decent amount of reverts, especially in comparison to the Christian faith where most are becoming agnostic/atheist, allowing Islam to quickly catch up to them and inshallah surpass them in 2050. Our reversion rates in the US are also pretty high, but it doesn't show because of the number of apostates who no longer identify with the faith (mostly Shiites from Iran).
You also have to take into account that the richer a nation becomes, the less religious it is as well. So far into the future the case could be made that all religion will be on the decline.