Yes, sensors accumulate charge as photons impinge upon the receptor sites. And then the sensor is read out or "sampled" one line at a time, synchronously. "Synchronously" may be the key here, because it implies the single "specific frequency" that Keith mentions in the thread, above.
This is a single drop of water, following the classic parabolic acceleration curve caused by the force of gravity on a projectile. But how the in-camera processing created it, I do not know. I tend to agree with Jonathan that it is an in-camera composite artifact. However keep in mind that, as regards sensor read-out, a frame is not an atomic entity. Thus, such an in-camera composite as Jonathan postulates might be comprised of only certain pixel rows or portions of pixel rows.