According to Wikipedia, the 2011 Portuguese protests were in part "inspired by Deolinda's song "Parva Que Sou", (English lyrics here ) talking about precarious working conditions Portuguese youth, in particular qualified university graduates, endure". Thus, once again, as in the Arab spring, in particular - events in Egypt - we hear of university graduates, and "precarious working conditions" which they have to endure.
Then, “Spain's May (2011) protests were influenced by the Portuguese protests, which in turn incited new protests in Portugal”. Thus, protests in adjacent countries, and around the world, influence each other, one igniting the other, and in turn further enflamed by the fire in the other.
What is the long-term cause of the protests in Portugal? Wikipedia mentions "economic crisis" in Portugal.