Poland to tighten border controls with Germany and Lithuania: What is the reason?

Poland will reinstate border controls with Germany and Lithuania to help combat illegal migration, states Polish Prime Minister Donald Tusk, according to Polsat News.
"We have decided to reinstate temporary controls on Poland’s borders with Germany and Lithuania. The decision was made today and will take effect on Monday, July 7," Tusk stated.
According to him, he had "repeatedly warned the German side" and "spoken several times with the new chancellor" — specifically about placing checkpoints located directly on the borders with Lithuania and Germany under special surveillance by security services.
As Polsat News reports, Tusk is expected to visit the Polish-German border soon to personally assess the situation with illegal migration.
Earlier this month, the Polish right-wing party PiS submitted a draft law to the Sejm aimed at tightening the rules for obtaining Polish citizenship for foreigners. The move was motivated by "migration threats," including a reported rise in crimes committed by foreigners.