The reason behind the fans' protest is UEFA's decision to donate €1 from each ticket to refugees seeking assylum in Europe.
As a result of the boycott Polish champion's INEA Stadion was practically empty.
Before the game started fans put a banner on stadium entrance that said, "Stop Islamization". Thus showing that refugees are not welcome in Poland.
During last home match fans of Lech Poznan displayed banners against refugees and chanted anti-refugees songs. At one banner you could read, "This is obvious and simple for us. We do not want refugees in Poland".
Source: World Bulletin