How can sugar and water go bad?

I make my own syrup. It consists of brown sugar, white sugar, maple flavoring and water. When the kids were little we went through it so fast I was making a new batch every 3 weeks.
However, since it’s just Rick and me, the batches sit for months and months.
The last couple times I’ve gotten into a premade batch, it’s gone “bad.” From now on I’ll just make what we need as we go, but I don’t understand how sugar and water can go bad.