There may be a few reasons for this:

1) maybe the grass on your field is rather high. I have a similar problem on our field. Mostly thing will be fine, but if the grass has not been cut recently and is a bit higher, I have the exact same problems as you describe. Putting on some bigger wheels, or moving /bending the landing gear a bit forward may help

2) Your landing gear is the far back, causing it to tip over. Perhaps it is mounted under the wing, almost near the CG.
change the landig gear mount then. place an extra reinforcement in the forward lower fuselage, and mount the landing gear well before the CG