Bordeaux's fifth defeat in six league games left them in sixth place on 57 points with four matches remaining, four points behind third-placed Lille who occupy the Champions League play-off spot.
"I do not know how to explain this," striker David Bellion told French TV channel Foot+. "Last year everything was fine for us and all of a sudden nothing is working any more.
"Maybe the defeat in the League Cup final and elimination from the Champions League played their part."
Bordeaux were top of the table seven weeks ago before losing the League Cup final to Olympique Marseille and being knocked out in the Champions League quarter-finals by Olympique Lyon.
Laurent Blanc's French champions, who were without injured playmaker Yoann Gourcuff, lacked inspiration in midfield and their defence was shaky.
Left back Benoit Tremoulinas was easily beaten in the air by Gregory Pujol who lobbed Ulrich Rame with a header after 12 minutes.
Bordeaux, a far cry from the side that beat Bayern Munich twice in the Champions League group stage earlier this season, failed to threaten a Valenciennes defence perfectly marshalled by centre back Dianbobo Balde.
Right back Fahid Ben Khalfallah set up Foued Kadir at the end of a sharp counter attack and he poked the ball home from close range for the second goal in the 72nd minute.
Blanc, who has been touted as a possible replacement for France coach Raymond Domenech after the World Cup, looked stunned on the bench, just like club president Jean-Louis Triaud up in the stands.
Bordeaux play their next game at home against Toulouse on Sunday.
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