I've seen this question asked many, many times, so it's not unusual. It happened to me, too.
The brain has two sides. The right side deals in images and the left side deals with facts like numbers, words, etc. So when you're looking at the images on Tarot cards, you're essentially reading in images not words, and to consciously translate that over to the other side of the brain where it becomes words is something you have to learn to do. For some reason, as long as that process stays in your brain and is translated in the left side, it happens more quickly than when you then have to accomplish a third step which is to put it into the spoken word. It's step 1--you "read" the images with the right side of your brain and then step 2--you transfer the images to the left side and give them words and then step 3--you take those words from the left side of your brain and speak them aloud in coherent sentences. It isn't something we're used to doing so it takes conscious effort till we get the hang of it.