I know some people who will roll the dice badly (maybe not on purpose), drop by mistake or over obstacles so it is likely to land cocked. They then re-roll it...unless it landed nicely for them in which case they don't re-roll it. Basically gives you a free re-roll if you need it.
Something I will watch closer for next time, maybe bring it up at the start of the game. Don't like to be confrontational, but also don't like to be treated like an idiot.