Explanation for this

Maybe you hit it too hard? Maybe your swing was upwards? Sometimes if properly hit, the underspin on the ball can become your top spin.

   No, it wasn't a serve, it was a volley, and that too while returning a fast ball.  It was a choppy looking ball.

With enough experience, you'll be able to correctly judge the amount of backspin on the ball and swing correctly.

Instead of a flat drive or low smash, against a somewhat high ball with backspin, try a topspin drive. Try a combination of blade angle and swing angle:

1. Swing forward with a closed blade face.
• Adjust the angle of the blade face, i.e., close it if the ball is still going long or open it if it goes into the net.
2. Swing forward and upward with a closed blade face.
• Experiment by increasing or decreasing the upward component in your swing.
• Experiment with the blade angle for each swing above.
  
• Find the right combination of of swing angle and blade angle that allows you to consistently make the shot.

The goal is to hit the ball more with the sponge and blade, as opposed to brushing it with the rubber, but still impart sufficient topspin to control the shot by swinging low to high

One good way to get a feel of the topspin drive I'm talking about is to practice countering about 3 feet away from the table. In order to do so, you'll automatically lengthen your swing and swing more low to high. Careful not to start looping, as that will make this drill pointless.

You can also practice this stroke by countering away from the table against loops, as opposed to counterlooping them with your own loop. This scenario requires a more closed blade face and a less low to high swing. Together with the previous drill, you should get an idea of how to adjust your swing angle and blade angle to suit the incoming spin.