不期然而然
happen unexpectedly; turn out contrary to one's expectations
戛然而止
with a grunting sound it stops (idiom); to come to an end spontaneously (esp. of sound)
油然而生
arising involuntarily (idiom); spontaneous; to spring up unbidden (of emotion)
知其然而不知其所以然
to know that it is so but not why it is so (idiom)
自然而然
involuntary; automatically
霍然而愈
to recover speedily (idiom); to get better quickly