Bipod on solid surface with nothing holding the forend is a recipe for what you are seeing.
One other factor to consider, I was shooting groups off the flat deck of my ute once, terrible groups, I didn't realise old mate was leaning and moving against the front end of the ute during the process, so if you have anyone along to 'help' when you are shooting off the back of your ute, make sure they aren't in the ute or leaning against it while you shoot. One other thing I have noticed about shooting off of the back of a ute, is that if the engine is running, especially diesel, it will affect accuracy. I doubt you were shooting groups with the engine running but I mention it to rule out all options.
If shooting off the ground with the bipod still doesnt fix the issue, try what Husky suggests, shoot off a solid sand bag. While not a great way to simulate hunting shots, it will tell you if the stringing is caused by shooter position or something at the loading bench. If you can confirm it is shooter position, at least you won't be chasing tails at the bench trying to troubleshoot a non-problem.
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