The angle valve under the sink is corroded stuck, leaks when you turn it, or is simply worn out. Replacing it yourself is a simple job — but most "kitchen water damage" stories start with exactly this kind of task, from following the wrong order.
The small valve between the main water line and your tap or dishwasher, letting you shut off water locally without draining the whole house. Essential for maintenance, but subject to wear and limescale itself.
Only closing the angle valve itself instead of the main valve — logical, since you're about to replace it. But a worn angle valve often no longer shuts off completely, and you only discover that once water sprays out anyway.
Find angle valves in the right size in our webshop. Not sure about the size or thread type? Send a photo of the old valve, so you're certain before you start.