How long you play chess and how often?
I think its the problem with reconizing patterns in chess, when I see Nb5 or Ng5 I immediately check c7 and f7 squares. Same as you see Qf3, Bc4 that kind of things.Everyone blunders from time to time if you want to reduce it you need to study some simple tactics (forks, discovered checks etc.) to familiarize urself with this kind of positions. Playing a lot of games can help you as well.
How can I improve on that problem? Here's an example from a recent game.