You might need to change your bit, or your horse might just be nervous about something in the area.
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 gelding was like this I bought a bit with a roller in it and helped.
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