obieone, Do you have any reliable statistics to support a significant rise in Belgian immigration? I've had a look and can't find anything (not saying you're wrong).
Wiki writes 'The largest group of immigrants and their descendants in Belgium are Italians, with more than 450,000 people, which is well over 4% of Belgium's total population. The Moroccans are the third-largest group, and the largest Muslim ethnic group, numbering 220,000.'
I know Belgium does have a large number of immigrants, but at least half of them are from the EU and many are temporary.
en.wikipedia.org/wiki/Islam_in_Belgium states that 25% of the population of Brussels is Muslim, but I can't find any evidence that there has been a sudden rise.
That still doesn't explain why right-wing thugs should target a vigil by ordinary Belgians, presumably of mixed faiths.