the footnote settings contain the Tab symbol between the number and the text. Yours is quite small, so when the number achieves 10 (2 digits), it becomes longer than the tab space. You have to change the Tab option in the Paragraph style which you use for your footnotes.
If anything here needs to be explained more clearly, just ask.
I can't quite tell what you have defined between the footnote number and the footnote text, but it is definitely not a tab and it should be.
You can control what appears in that space via Type > Footnote Options. Once you remove whatever is there and replace it with a tab, you can set a left-align tab that will offer consistent spacing.
Did anyone figure out what that character is? I'm too lazy to hunt down my hidden characters glossary.