How is this a blunder!?

Rxe7 is easily winning because the White queen is placed in a very dangerous position for the Black king. If the game follows 1...Rxe7 2. Rxe7 Qxe7 3. d6+ Qf7, then 4. Bd5 finishes the queen.

ajl721x, if the Black queen captures the hanging knight on b6, then Black's doom will come even more quickly i.e. 1. Rxe7 Qxb6? 2. d6+ and it's sayonara for Black.

The main reason why the computer labels this a blunder is because the best move available is better than your played move by many points, even if your played move is still winning. I would personally play the same move 1. Rxe7 rather than the best possible move 1. Re6 (which technically looks like a mouse-slip of 1. Rxe7).

P.S. I wonder how the White knight managed to land on the b6 square.

But to my understanding a blunder is a game losing move, right? shoulding this be called a mistake instead?

That is not how a blunder is defined in the auto-analysis software I've seen.  In the ones that allow you to define whether to use ? or ?? (mistake or blunder), the difference is in how many centipawns the move differs from the "optimal" one.  I've seen mistakes defined anywhere from fractional to full pawns, and then blunders being anywhere from most of a pawn to a pawn and a half to a piece.

I don't know how chess.com defines a blunder vs a mistake, but my educated guess is that it is defined in terms of scoring change and not game winning/losing.  That, in itself, would also require some type of boundary.

It will depend on the depth that it's running.

You are essentially inviting a trade of 2 of your rooks for a rook and a knight. I would probably just have retreated the knight to A4. Or rook E6 to cover the knight from being taken by Blacks queen. Either way, save that knight.

It is not a blunder. Just making the win "harder" on the computer level.

For example, when you can force a winning pawn endgame, you may simplify the position but computer may say blunder anyway if you had a checkmate in 9

what if after Rxe7, black move: Nd4? black wins the Knight and the Bishop and get better position, that why is a blunder, white loose position and material.

I don’t understand what you are saying...

white is up a piece and the bishop is protected by the pawn,the knight can get protected by the rook

white is still up material

what if Rxe7, Rxe7, white getting the Knight.

After that white will play Rxe7, if blacks Qxd7 will end up loosing the Queen because the bishop. so instead playing Qxe7, blacks plays Nd4, it may prevent Re6.  as you say is right, trying to obtain the rook or the Knight will be a disaster for black if that pawn moves, putting the black King in check, because the Bishop and the Queen.

erm so isn’t white just up a piece?

Being more respectfull and polite, black have got a better position than trading the Queen for the Rook in a inminent check mate. This movement worsen the white position at all, it is true there's still a knigh up, but if white comes with another blunder may end up loosing the game, til this point having a  knight up is better than having a queen up on the proposal movement Qxe7, wich is clearly another blunder.

it is classified a blunder because white loose all the advantage, also might end up loosing more material than blacks, position and maybe the game.