The following is based on a large amount of personal experience.

With memory cards almost anything conceivable can happen, and sometimes does. If there is an approved means of shutting down a memory card before removing it from the camera or computer AND you care enough, then you should use it. I almost invariably swap cards in both computer readers and in cameras without ejecting them or powering down the camera, and I almost never experience any problems as a consequence (with probably somewhere in the ~~~ 10,000 range card removals over many years) BUT it is always a possibility.

The most likely reason in your case is that the card directory structure was "open" at the time of removal in such a way that the card removal left it in an illegal state such that the information for the new files was invalidly formated and so was ignored. It could also have been "happenstance" based on a system glitch (power spike, alpha particle event*, ...) that just happened to coincide with the card withdrawal, BUT this is less likely. It is more likely that you were the author of your own misfortune. At a minimum, if there is a "write in progress" indication from your operating system (unlikely), or card reader/writer or camera (usually a red light) then do not remove the card. Data may be corrupted if the computer has written it to a buffer but the buffer has not been "flushed". Some systems offer the option to disable buffering of memory writes to selected devices and doing this will often increase data security at the possible expense of increase transfer times. I disable buffering where possible.

If you care about data integrity from portable devices then, to the maximum extent possible ensure that data transfers have been completed before taking actions that may compromise data, copy & retain rather moving (= copy + delete indivisibly), make a backup copy of copies data before deleting the original.

---

*Alpha particles are generated within the encapsulation material of memory or other electronic devices by cosmic ray events and very occasionally can and do cause 'bit flipping" in semiconductor devices. Occasionally these make Murphy's day. Modern devices are generally designed to minimise the effect of such events. An especially energetic cosmic ray may in due course help you to lose photos :-).

---

===> Wikipedia 'Cosmic ray' notes - see section"Effect on electronics""

• Cosmic rays have sufficient energy to alter the states of elements in electronic integrated circuits, causing transient errors to occur, such as corrupted data in electronic memory devices, or incorrect performance of CPUs, often referred to as "soft errors". ... This has been a problem in extremely high-altitude electronics, such as in satellites, but with transistors becoming smaller and smaller, this is becoming an increasing concern in ground-level electronics as well.[26] Studies by IBM in the 1990s suggest that computers typically experience about one cosmic-ray-induced error per 256 megabytes of RAM per month.[27]

• To alleviate this problem, the Intel Corporation has proposed a cosmic ray detector that could be integrated into future high-density microprocessors, allowing the processor to repeat the last command following a cosmic-ray event.[28]

• Cosmic rays are suspected as a possible cause of an in-flight incident in 2008 where an Airbus A330 airliner of Qantas twice plunged hundreds of feet after an unexplained malfunction in its flight control system. ...