A lot of times, when my cat is in the garden, I've seen him eating blades of grass (maybe just chewing them, I'm not sure if he swallows).
Why does this happen? Is it normal?
Yes, it is normal! As someone said before it is frequent that cats do it for cleanse as their natural instinct tells them to do so.
However, from my experience, I can say that it is normal as long as it doesn't happen very frequently. If it happens like everyday on a regular basis it may be a problem and you should have your cat checked by a professional. Cats eat grass for cleanse but if its exaggerated it may indicate that the cat is having any related stomach/digestion issues. I would suggest that you take a closer look to the situation and see if your cat does it on a regular day basis or just a few times sporadically or every time he's in the garden - for this last one, if the cat isn't allowed in the garden that often, it shouldn't be a problem as the cat is eating grass when he only has access to it. Other things to look into, would be:
Again, I repeat, this is from my personal experience and if nothing from the above applies then I believe your cat is just fine!
Never the less I "googled" a bit and come up w/ these links that I found to be quite helpful: