It is how the Autocorrect command to capitalize the beginning of a sentence works. You can switch the option off, if you want. Ctrl-Z (undo) will uncapitalize it.

By Autocorrect's algorithm, a full stop (period) followed by a space signals the beginning of a new sentence, just as pressing the Enter key starts a new paragraph and sentence.

For usage, I think you mean first place rather than one place.