You don’t just get off a boat and start committing crimes. You have to be a wrongun in the first place. My theory is that many of these men are on some kind of wanted list back home. Maybe they’ve pissed someone off, maybe they have been threatened with the law or someone is after them for attacking their daughter or sister. Maybe their lives are in danger back home and their option is; get your legs broken or be killed back home, or flee to Europe.
That, or people are opening their jails and pointing them in the direction of Europe as a way of getting rid of the problem. Or families are sending over their problematic members - get them out of the way plus possibly a remittance sent back if you're lucky.
I've also read that Putin deliberately sends foreign criminals to destabilise Eastern Europe.
It seems plausible to me - I mean I've lived in the Gulf where citizens are minorities in their own countries, yet crime rates overall are extremely low. Why? They allow in the cream - hard working, law abiding people who aren't drawn by soft-touch laws and generous benefits - and we get... well, the opposite of that.