I prefer to stack two rows together on pallets or logs or what ever, the tend to help brace each other. I had very few stacks fall over, most notably many years ago a brother in law stored his boat down back and caught the edge of trailer on the stacks on his way out and totally wiped it out. What an idiot, thankfully he is no longer in the family.
Well I was VERY careful restacking this weekend and one of the same ones fell again, I think the culprit is uneven ground. I had not noticed until today. It will be corrected proper this week and restacked.