AFAIK, they just didn't want to get dragged into another war by the Germans, and perceived the joint UK/FR/BE war plan as provocative towards Germany, and left it to proclaim their neutrality. Unfortunately for them, proclaiming their neutrality didn't save them in 1914, and it didn't save them in 1939 either.
Now I totally understand why they were not fond of FR/UK deliberately planning to sacrifice half their country to establish strong defensive lines on their rivers, but sometimes you can't have it all and just have to go with the less shitty plan.
I’d imagine that the literal millions of dead under Belgian soil, and the total destruction wrought by WWI made them hesitant to take any sides in potential conflict. You don’t position yourself in the middle a fight between two heavyweight boxers when you are a flyweight.
Now I totally understand why they were not fond of FR/UK deliberately planning to sacrifice half their country to establish strong defensive lines on their rivers, but sometimes you can't have it all and just have to go with the less shitty plan.