Can someone help me understand why this move was a blunder?

Hi everyone.  I'm new to chess as a hobby and have been battling against the 1200 score adaptive AI (which still generally beats me).

Usually when I look at the analysis afterwards I can see why the analysis tells me that some moves were mistakes or blunders. Like "Oh yeah, of course I should have played that instead..." or "Oh yeah, I totally missed that threat". But in my latest game the analysis tells me I threw the game away with a blunder and I can't see what's the issue. Clearly it was a bad move because I lost the game but I'm trying to figure out how to understand it and avoid in the future. So I was wondering if someone could help me reason about it?

So the board was this and up to this point the analysis thought I was strongly favoured to win the game

Then I played Rxf3 taking the pawn. This the analysis classified as a blunder (saying Kh7 is the best) and the white immediately became the more likely winner.

I can sort of see why protecting the king would be a good move. White is clearly setting up their rooks for attack (and that is how I ended up losing a while later, mated by two rooks)...

But my move still looks good to me. It takes the rook away from being threatened by the queen, it wins a pawn, threatens another pawn and threatens a checkmate on the next move.

Is it just that in this position, doing anything else than moving the king is disastrous and almost anything else would count as a blunder? Or is it that the move I did was specifically worse than some other options I would have had?

Thank you for your input!

In the position posted you are down an exchange (rook vs bishop) but have an extra pawn and strong threats based on your far advanced pawns on e3 and c4. This is good for you because white is not yet in a position to attack you and your threats are coming very fast. However, this situation will not last forever. If you fail to tuck the king away on h7 white will play Qxc4+ forcing an exchange of queens. After this it is whites extra material which will prove decisive as your pawns can now be easily stopped and the white a pawn and pair of rooks are very strong. If you go Kh7 white cannot capture on c4 because Qd2+ will lead to mate for you. If you take the precaution of tucking your king away all of your threats are renewed like Rxf3 threatening mate on f1. 

Thank you to both of you!  Yeah, the forcing queen trade is pretty much what they ended doing and it was fast downhill from there, but I guess I hadn't realised that they couldn't do that with the king in a bit safer position.

Welcome to chess! A wonderful, but very hard game

Yes often these kind of moves are the hardest to find. When attacking it feels like you can't waste any time because the opponent will find a way to defend or you will lose your chance to win material. But often in the midst of an attack it is necessary to play a quiet or precautionary move which allows it to work, either because you slowly bring another piece into play or like here you stop any chance of a counterattack. 

Black is clearly better.  Best seems ...Rc8 if now Rd1 Bc3+ wins. And if the White Q doesn't stay on the e1-a5 diagonal then ...Qd2+ and mate next move.  After ...Rc8, you're threatening ...Bf6 then ...Bh4 mate which is virtually impossible to stop without White losing material.

I'd also say you don't want a queen trade with the check, you lose a pawn also and what I find most important is, your queen was a great attacking piece here with white's king very vulnerable and as you said with a potential mate looming, your king was pretty safe in the corner especially with one more move to h7, maybe the rook move would have even been potentially devastating on the next move if your king was on h7 and depending what white's next move had been. I think there could be some other moves that maybe aren't disastrous - disastrous for me assumes your opponent makes the best move. For example, move your rook to c2. Maybe you lure your opponent into checking you on the back rank with the queen, move your king then to h7, maybe your opponent is distracted enough thinking about attacking and loses concentration of defending his/her king and fails to compensate for the queen no longer defending and you for example manage to check with your bishop and win the rook because they are forced to take

you should go back to the analysis. it will tell you what the best move was. Doing the analysis is a good idea. The analysis helped me a lot when I started to get serious about chess. when I was a 400,  I loved ladder mates. One day, I went to the analysis and saw a shorter mate. The great part about chess is that the same or similar positions happen in different games. So if I mated someone one time, and now we are in the same position, I know what to do.