Why do I blunder after my opponent blunders?

Its not specific to one game, its a definite pattern I see. I play games online here, afterward subject them to the engine's analysis, and it so often happens that following my opponent blundering, I blunder. I have no idea why? Any thoughts or experience with this?

I took a look at your last 5 losses, and i don't see what you're referring to.  I'm not saying that its not happening, but that wasn't the reason for your last 5 losses.  What i did see, was the usual:

Not following opening principles.

Hanging material.

Missing simple tactics.

Play slow time controls, and use the following:

Opening Principles:

1. Control the center squares – d4-e4-d5-e5
2. Develop your minor pieces toward the center – piece activity is the key
3. Castle
4. Connect your rooks

Tactics...tactics...tactics...

The objective of development is about improving the value of your pieces by increasing the importance of their roles. Well-developed pieces have more fire-power than undeveloped pieces and they do more in helping you gain control.

Now we will look at 5 practical things you can do to help you achieve your development objective.

They are:

1. Give priority to your least active pieces.

• Which piece needs to be developed (which piece is the least active)
• Where should it go (where can its role be maximized)

2. Exchange your least active pieces for your opponent’s active pieces.
3. Restrict the development of your opponent’s pieces.
4. Neutralize your opponent’s best piece.
5. Secure strong squares for your pieces.

Don’t help your opponent develop.

There are 2 common mistakes whereby you will simply be helping your opponent to develop:

1. Making a weak threat that can easily be blocked
2. Making an exchange that helps your opponent to develop a piece

Pre Move Checklist:

1. Make sure all your pieces are safe.
2. Look for forcing moves: Checks, captures, threats. You want to look at ALL forcing moves (even the bad ones) as this will force you look at, and see the entire board.
3. If there are no forcing moves, you then want to remove any of your opponent’s pieces from your side of the board.
4. If your opponent doesn’t have any of his pieces on your side of the board, then you want to improve the position of your least active piece.
5. After each move by your opponent, ask yourself: "What is my opponent trying to do?"

The reason is because the computer sees a way to punish their blunders and you happened to miss it.

The difference in evaluation between your move and the computers is so big that it counts it as a blunder even if the move you made is okay. Don't sweat it. Consider blunders to be when you hang a piece or pawn, allow your opponent to penetrate your position, missing a mate in 3 that you should of seen, etc... 

You can't possibly see everything that the computer sees. What you are talking about happens in most of my games as well. Sometimes the computer lines give lessons if you study them, sometimes it is just a very computer-like line that is done just by sheer calculation power and really isn't understandable at all.

It's called "Blunder Echo" When your opponent blunders the wave forms of the blunder energy interact with your decision making in a synergistic way to amplify your blunder energy. Then you blunder too. It's a common phenomenon. Unfortunately it will not work for checkmate however. Chances are that whenever your opponent has checkmated you, you did not checkmate them on the next move.

 It means you didn't take advantage of your opposition's blunder. example, If your opponent hangs their Queen and you play a causal move instead of taking the Queen, then it will  be a huge blunder from your side too..

This "double blunder" phenomenon is very common... even at the World Championship level.

Magnus Carlsen vs Levon Aronian, Sao Paulo 2012

fantastic answers, thank you everyone. 

Simple: Get good. Tactics are your friend and any hope of the Scandinavian should be eradicated immediately.

Be careful about what you say.

Have you ever "blundered" mate? I have. I get a bigger surprise than my opponent on such occasions.

Blunders can sometimes change the game in very dynamic ways, sure your opponent made a blunder, but it may have also given him opportunities he didn't have before. For example you win a piece, but now he has a pair of connected passed pawns, objectively your better, but with inexact play you could easily lose.

Two of your pieces of advice, i.e. to Improve my least active piece, and stopping to make threats that are easily countered... are already helping me. Thank you, and thank you everybody.