These past two years have been really narrow decisions. Two years ago, they gave it to the skater with the most shocking, gnarliest, and most death-defying part (David) rather than the gnarly guy putting out constant footage (Figgy). It would've made sense to give SOTY to either of them, but at the end of the day, only one guy can get it. Giving it to David definitely makes sense and is certainly not an unjustified decision.
This past year, they did quite the contrary and gave it to the stylish ripper putting out multiple parts (Ishod) rather than the skaters with the gnarliest single parts (Westgate and Nyjah). Again, either of these three would've made sense (although I'm personally glad Nyjah didn't get it).
For this reason, I can't really see any pattern in their choices. Thrasher has given SOTY to all kinds of skaters.