The demonstration was in Germany
Demographically, Turks constitute the largest Muslim population in Germany.
From the CIA World Factbook:
Ethnic groups:
German 91.5%, Turkish 2.4%, other 6.1% (made up largely of Greek, Italian, Polish, Russian, Serbo-Croatian, Spanish)
Religions:
Protestant 34%, Roman Catholic 34%, Muslim 3.7%, unaffiliated or other 28.3%
It stands to reason that more Turks would be protesting in a place where there are more Turks.
If we are going to look for more Arabs, we need to look to France, with its large ethnic North African population:
Ethnic groups:
Celtic and Latin with Teutonic, Slavic, North African, Indochinese, Basque minorities
Religions:
Roman Catholic 83%-88%, Protestant 2%, Jewish 1%, Muslim 5%-10%, unaffiliated 4%
CIA World Factbook