I had something like that happen to the end of a row of cabinets in my kitchen. It was from the coffee maker over flowing. Coffee ran down the side & caused the damage. The damage was along the bottom edge & we covered it with a piece of baseboard.

Is there a lot of weight on it that is causing the cabinet to buckle? What kind of countertop do you have?

That is water damage. From somewhere. Is that an exterior wall? Are you on a slab foundation with pipes in it? Does the wall have pipes in it? You have got to sleuth this. Water can run a very long way.

It could be that a water line runs across the ceiling in attic to the refrigerator and it is leaking. Or depending on where the refrigerator is located it could be an over flowing drip pan. I had this happen and the water traveled under a pantry and under a built in desk in adjacent room.

We had years ago something similar in the washing room...it was also a water damage and we had to partly change/repair it. As krissie commented...the water source can be something else, that you maybe don't know.

We had that happen and it was the dishwasher pump had a hairline crack so it only leaked a little when it was run- not all the time. We discovered it when the hardwood floors cupped...

This was also in our case...the water leak was a very small one and we didn't actually noticed it until we saw the damage on the cabinets. I would have preferred a bigger water damage so I would have seen it before and possibly prevented the damage on the cabinets. Sometimes small water leaks are worse :)