If you mean lagging (constant jumping) or freezing (screen and music tone get frozen), it is most likely beacuse you hacked the game, it is glitched, or you are using a ROM or Emulator. These are abnormal conditions to the game, the the game may lag or freeze upon a certain action, in this case, an evolving Nincada. This also depends on the game you are using; some are glitchier than others, paticularly Gen I, II and III. Game Cards/Game Paks may be dirty or moisturised, and this is another reason why the game may glitch up.
Glitches are anomalies in software programs, such as video games. They can cause various problems ranging from the purely graphical and audio glitching to completely corrupting or deleting the save file.
Glitches are mostly caused by problems with a game's code, or players doing things the programmers didn't anticipate, therefore causing the game to react unexpectedly.
~ Bulbapedia's description of a glitch