According to polls, Parvanov won 64.0% of the vote, with a 40.0% turnout.
64% * 40% = 25.6% of those eligible to vote have voted for Parvanov.
When Parvanov would have had 50.1% of the votes, with a 50.1% turnout:
50.1% * 50.1% = 25.1% of those eligible to vote.
Conclusion: a second round is necessary because not enough people voted AGAINST Parvanov.
Weird election law...
No, the second round is necessary because according to the constitution,
a candidate wins in the first round only if in addition to having over 50 % of the votes, at least 50 % of the electorate has voted. So there would be a second round even if Parvanov had 50.1 %. This law, by the way, is very common in eastern Europe - it is probably believed that if there isn't at least 50 % turnout, the election results won't be taken seriously.
About the results - they are not unexpected, but Parvanov's extremely large victory margin is. He actually won
all municipalities in Bulgaria!