There may be several reasons for this:
Your iron produces a lot of steam. Steam could have condensed on the ironing board cover, making it wet and cause water marks on your garment when the iron is pressed on it.
Ironing aids or fragrance water could have been added to the water tank. This can cause problems with the steaming function, causing water to leak. Use only tap water or distilled water or a mixture of these two types of water.
Steam setting is being used at too low a temperature - The steam function should be switched to "no steam" when using the iron at lower temperatures or the water tank should be emptied.
Steam surge/shot/spray being used too rigorously - try using these facilities more slowly.