Somehow, the planters are either watertight enough to be a pond or contain soil that is allowing the planter to trap water. Here are your two potential problem areas:
-What type of soil did you add to the planters? If it was regular garden soil, that might be the problem (too much clay). Store soil is not usually an issue.
-Another is the planters themselves. If they are built too well, they could have ended up watertight. If the planters have a bottom you are going to need to drill holes in the sides to let the water out. If the planters do not have a bottom, they are too flush with the ground. Lever up the corners a little.
Please let us know how it goes.