why do you think deep sea creatures would come out of the surface?

The water mass they've been living in got displaced toward the surface. Usually caused by breaking internal waves, although earthquake-caused underwater landslips can do it. The critters that live in the displaced volume have lived in a very homogeneous environment all their lives and don't really have a way of knowing what's changed or what to do about it, many of them have buoyancy control mechanisms that are overwhelmed by the depth shift.

Sometime the sea water will drop for some reason and trapping many sea creatures in bays. It's just an idea.

mermaids

My guess (and it is just a guess) would be that something changed in their normal deeper-water environment that made them flee. Low oxygen, change in temperature, maybe even new predator(s). Unless perhaps we are talking only one species of creature, in which case maybe it is mating season. I doubt that many species would all mate at the same time.

Lord......... sounds like they're planning for some massive earth quake!