As you've guessed, and others have mentioned, it could be a lot of things.

With taildraggers, it can also be the result of a poorly located CG. When the tires touch the ground the mass of the plane tries to keep going in the direction it was going, toward the ground. If the wheels are located properly in relationship to the CG, they absorb the landing shock in such a way that the pitch of the plane isn't affected. If the mass of the plane is too far aft that the mass has some leverage, that mass keeps going toward the ground. The gear keeps the front from going too. The pitch is changed. The plane's angle of attack pitches tail down. The wing has been forced to provide more lift than you had established and that was giving you the slow descent you had right up until the tires hit. The plane starts to climb in response to that new increased lift.

So how can that happen with our models? The designers worked out the location of the gear to the CG to give good manners landing, right. Well, they also worked out the location of the CG for flight manners too. You did work out exactly where the CG is in your model, right? And it's in the range the mfg suggested, right. And you've not had to repair damage and might have somehow added weight aft, or (I've seen this more than a couple of times) put the landing gear back on backwards, right.

It could be the CG. It's not often the case, but look at the model again and maybe just give the CG a look. Sometimes they change. Sometimes the location of the wheels change.