When my horses do this, it tells me they’re relaxed. However, if your horse is chomping on the bit and making you unseated, make sure your contact and seat are strong. Use your legs to push your horse forward and hopefully this will make your horse stop chomping on the bit.
My horse does this to but only when I have little to no contact to but that is the only time she will do it and I believe it is because she is bored so try doing more things to keep her occupied!
Hope this helps