The leader of the S&D group in the European Parliament, Hannes Swoboda, says that the move came at the request of the Romanian Prime Minister (and PES member) Victor Ponta, but the change of venue is being interpreted as a result of political pressure and a desire of political leaders not to be seen with a government that's had its commitment to the rule of law so seriously questioned over the past few months.
"[Socialist sources from the European Parliament], who asked not to be named, said “only a couple of leaders” of the PES-affiliated parties across Europe had confirmed attendance at the Bucharest congress, the rest apparently being reluctant to be hosted by Ponta, who has been under fire since he took office this spring."
The loss of the congress may also be a blow to Ponta's image and electoral strategy for the autumn election:
"When the decision to hold the congress in Bucharest was taken last year, Ponta was in opposition. As Romania’s parliamentary elections are due in the autumn, the congress was designed to boost his international image ahead of the poll."
So EurActiv is (indirectly) calling the news of Ponta's request out for what it is: a face-saving exercise:
"PES sources said that while leaders of centre-left parties didn’t want to be criticised at home for being hosted by such a controversial leader as Ponta, there were also concerns that he should not be weakened before the election.
It would be a sharp blow for Ponta, who will turn 40 on 20 September, if the congress is moved away from Bucharest, a Romanian PSD source told EurActiv.
In any case, it appears that a decision to move the congress to Brussels should be coupled with a face-saving reason to be used internally in Romania, a difficult exercise for the PES secretariat."
This shows that the PES is much more uncomfortable with its Romanian member's actions than its weak response to the political crisis on the floor of the European Parliament would suggest.