I think, in some cases, especially in areas where homeschooling isn't "common", there are the following issues: (not all these are *me*, but they are true to *someone*)
1) surprise at seeing a school-aged child out and about in town during school hours. I used to live in an area where truancy was an issue. Many local stores and businesses actually had signs up stating that children from 7-17 were not allowed in there between 8 am and 3 pm Monday through Friday unless accompanied by a parent. (7 was the start age for mandatory education). If a child was waiting outside the bathroom for mommy, and security saw her, then they might start agressively questioning her as to why she isn't at school. Some parents just did not care whether their child was at school or not. A lot of kids had a habit of cutting school.
2) the local public school is so great, why don't you put your child in it? My child was reading before the end of the first semester of kindergarten. They get to do all kinds of stuff at school. --the town DH is from. One school. Many awards on all kinds of things-sports and academics. Children left kindy reading. Everyone attends. People in nearby towns try to get their kids in there--to the point the principal can pick and choose who gets in.
3) I'd go bat-crazy if I had my kids at home all day. --parent/child personality fit? introverted low-energy parent with extroverted chatterbox whirlwind child?
4) I'm looking forward to putting my kids in school so I get time to myself ("free daycare" whoo-hoo!) --I can get stuff done without someone undoing it just as fast...
5) Homeschooling--all the options and such, just confuses me. I'll put my kids in the local public school so I don't have to think about it.
6) I work. My partner works. We don't live near family and don't have friends that can take our kids during the day while we work. All we can find are before-and-after school programs, not all-day care after preschool age. Opposite shifts are not an option in our career fields and we cannot cut out one income--it takes both to live.
7) Some believe that you are only homeschooling because the public schools are bad and you can't afford private (look at #2, especially if you live in an area with a good public school)
Rather than jumping all defensive, assuming the public schooling mom is dissing on your choice, ask her 'What do you mean by that?' I honestly think both sides "reasoning" for what they do can sound "defensive" or "hostile".