It might be to do with a feature called Smart Punctuation. If it's turned on (which I think it is by default) then when you type a double-quote mark ("), it turns it into curly quotes (“ or ”, depending on whether it's before or after a word). You might not be able to see the difference in this comment, due to the font used, but
this article shows the difference.
This might not be what's causing it in your case but when I turned on Smart Punctuation a moment ago and tried adding
“silver birch” to one of my photos, Flickr added the tags
“silver and
birch” as two separate tags, each with a quote mark before or after it. If this is what you're seeing then you can go into Settings, General, Keyboard and turn off Smart Punctuation. I don't know why it's the default as only writers are likely to use it.