SNP quality distribution peaks at 222 from variant call pile

[elfuser]

I have reference mapped paired end illumina reads and called variants using BWA and Samtools respectively. The resulting vcf was treated to remove high coverage SNPs with

vcfutils.pl varFilter -D30

and then filtered for low quality SNPs using awk

'($3=="*"&&$6>=50)||($3!="*"&&$6>=20)'

I graphed the distribution of SNP quality and observed a huge peak at 222., I repeated it with other samples and observed the same peak. Any clues as to why I may be seeing this?