Why and how? Already explained - if you don't accept it ... up to you.
Moreover, unless this 'fast track' procedure has compromised security what's the problem? Security is part of the problem with any accelerated vetting.
The graph shows the rising trend in recent years. Is the cause of that not and human trafficking assisted by free movement within the Schengen Zone?
If a border is open it is, by definition, easier to cross than one where controls operate. However, Germany has had an open border with all its' neighbours since 2007 - and to the West long before that - which predates Merkel's relaxing of restrictions. Again the numbers in recent years, especially since 2015, are far higher than in 2007. No real comparison.
I strongly suspect that those attempting to reach the UK do so simply because it is the only country - for whatever reason - where they want to settle. To that end, for these people, what measures are taken in France, Germany or elsewhere is largely irrelevant; they will continue to try to reach the UK. Yes, especially while traffickers are allowed to operate in the EU.