Maybe due to off season, without realising it, your grip has changed so the seam is coming out differently.
I had something similar happen to me so I bought some of the red/white balls so I could see what was happening a little better.
Could have also been a knackered ball. No seam, been knocked out of shape so it's out of balance. Sometimes you can blame the tools. I can't bowl offspin but I flip the ball up like a offspinner to see if the seam stays upright.