I think it could be a number of things.
1. The rising agent. It could be too much or too little as said before. This is one ingredient you do want to be as fresh as possible as the rising agent defintiely has a shelf life and will affect how your baking rises.
2. Also as indicated before it could be the fact that you are opening the oven. Are you opening 5 or 10 minutes before the time? Generally that isn't a problem. The reason I say it could be a problem is that if you open the oven door too early, it could be a t a point where the starches and the protein are forming a structure and thus collapse.
Let me explain it like this. A cake is made of starches (flour) which are like little levels of your cakes. The proteins are your pillars or re-inforcements which give it the structure, they hold the cake together. And the fats and sugars fill in the levels with the most important ingredient - air.
If the oven temperature is too low and your rising agent provides a POOF of air, but your starch and protein have not formed a structure due to low heat, then that cake is going to expand but because there is no structure it is going to fall .
Test to see if your oven is in fact the correct cooking temperature. HAve you been having uneven baking lately?
3. Okay so your rising agent agent was enough and fresh, you only opene it 5 minutes before the time, but it still fell flat? Did you cream your sugar and butter. Doing this will add to the aeration. Did you whisk the eggs before incorporating lots of air, but you did not overbeat them? Did you fold the dry and wet ingredients in carefully, making sure not to beat out any air. Yes, these techniques matter because it makes your cake light an fluffy!
4. I cook in glass tins. The reason I do this is because the batter cooks much faster in glass or dark baking tins. If your tin is shiny, it is going to reflect radiant heat, and cook slower than another cake would.
5. The size of your tin. Can the structure of your cake sustain the height or the breadth of the cake? IF the cake is too wide or too hig, the structure might not be able to sustain it. Remember size matters.
6. An lastly, I often used to just pull the cake out of the oven and not wait until it was completely cool before I turned it out. The structure of the cake is still unstable while it is hot. So, wait and let it rest and cool before turning it out. IT may collapse on you just because it was not ready to be turned out yet.