IMO, this mainly happens when you become aware on some level how dreams feel in comparison to real life.
QFT. Most of my lucid dreams start with an abrupt realization that I'm dreaming. I don't do a reality check; I just realize that I'm dreaming because the dream-world is so much different than the waking one. I'm semi-lucid most of the time anyway (I realize that I'm dreaming, but I play along with the story that's in front of me, or I don't fully realize everything I can do in a dream).

Remember that reality checks are just one tool at your disposal when it comes to lucid dreaming, and they're especially useful for deliberately inducing a lucid dream. There's nothing strange about becoming lucid without going through a reality check first. (And if it is strange, it's also awesome.)