Why Blunders Happen
Personally, I think the biggest factor in blunders is a lack of objectivity. As humans, we have a very real, natural bias in our own favor that prevents us from evaluating ourselves, or our work, objectively. Ego, overconfidence, however you might call it--these are the things that blunders are made of, because at heart, a blunder is just some move that fails to account for a threat. In my experience, this always boils down to one of two things:
- My opponent had a threat that I didn't see.
- My opponent's threat was more significant than I realized.
The specifics as to why I didn't see their threat, or didn't consider it well enough, may vary slightly, but the root is always the same: for whatever reason, I didn't consider the board objectively enough. I was too wrapped up in my own plans to even consider that there might be ways to interrupt them, or I figured that my position was so strong that I could play carelessly, or I was fatigued and couldn't focus, or I was under time pressure and didn't have time to consider the position as carefully as I should have.
Some of these underlying factors are out of my control: time pressure and fatigue, especially, affect even the greatest players*. But the others, I can train myself to handle, because they're all essentially the same thing: for some reason, I thought that I could get away with not evaluating the board thoroughly from my opponent's point of view.
The Single Best Blunder-Reduction Habit I Know
Since you play mostly electronically, I recommend you make use of one of the most powerful features of that medium: the ability to view the board from your opponent's seat. Almost all chess software (both online and offline) provides the ability to "flip" the board.
Every time you're considering your move, get into the habit of flipping the board and viewing the position and your candidate move from the other side. Try to come up with the best move for your opponent. This is where understanding your opponent's ideas comes into play, but if you don't know what he's doing, just try to find the best move as if you had his pieces. If you're stuck for where to start, look especially for threats created by his previous move.
Really try hard to find a strong move: there's this temptation to slack off when looking at the board from the other side, because you feel like you're betraying your own position or something: But if you can find a winning move for your opponent, that's awesome. You found it before you allowed it to happen, and now you can choose some other line!
Nine times out of ten**, this strategy will reveal any serious flaws in the move you're about to make. As you play more and your visualization improves, eventually you'll be able do this without physically flipping the board, and once you get used to the thought process, I strongly recommend that you try to wean yourself from the visual crutch of viewing the board from the opposite side, since you can't do that OTB. But as a beginner, the visual aid can be a huge benefit, and it helps to break the "I don't want to play against myself!" mentality.
*Not being a great player myself, I can't speak to what other pressures they may face.