Is it definitely thinning hair or is it just losing more? It may not sound like there is a difference, but there can be.
It's okay to be losing more hair, as long as it's being replaced.
When I went vegan I noticed that I was losing more individual hairs everyday, but my hair is much thicker overall because I have so much more new growth. Even my hairdresser commented on the amount of it - she'd never seen anything like it before. An easy way to check for new growth is to push your hair back from your forehead - the more hairs you have that are shorter than full length, the more new growth you have. What you see there is happening all over your head. Mine is almost ridiculous - I have to brush it back so it doesn't look like I have a fringe.
It's normal to lose about 100 hairs a day.
I eat a brazil nut everyday because it has about 150% RDA of selenium, and I've heard that is good for hair and nails, plus lots of fruit, veggies, whole grains, nuts and protein. I still eat a good proportion of junk food too, but never at the expense of nutrients.http://www.cyber-nation.com/livingst.../morehair.html
I don't know how reliable this site is, but it claims that copper plays a large role in hair loss. Too little and you lose hair, too much and you lose hair. It also claims that vegetarians retain more copper than meat-eaters, so it could be that those losing hair are intaking/retaining too much copper.
Googling on the subject, it appears that lack of B12 and zinc are the biggest culprits, but I also found this interesting article that may shed some light, especially if those experiencing hair loss were new vegans : http://www.vegsource.com/articles/harris_hair_loss.htm
ETA: I did a bit more reading about telogen effluvium as mentioned in that last link, and it does sound like this may be what you are describing. Luckily, it's short-term and doesn't seem to have long-term effects. It's likely that, in the case of the newer vegans, they weren't eating properly, transitioned too quickly or reduced their calorie intake drastically. It occurs when the body experiences a 'shock'. There's plenty of further info if you google it.