Well, it is just what you think it isn't - driver flex, caused by the diaphragm. Although the LCD-X is an open-back planar, its drivers create a seal (that's probably also one reason for their awesome and lively bass body), and when you press the headphones against your ears, the pressure change deforms the diaphragms. It also happened with mine a couple of times, but didn't do any harm at all.
As a side-note, some IEMs (especially closed dynamic IEMs, but also open dynamic and a few closed Balanced Armature IEMs) also "suffer" from driver flex during insertion, which hasn't done any harm except for very few known cases (when people pushed them too fast and hard inside). Don't worry too much if the sound signature or quality has changed (my LCD-X is still fine although I have done this torture at least 2-3 times).