It may be caused during the course of clicking around among and between
tabs that you accidentally press the Shift key. You can make it happen
on purpose by pressing Shift and clicking a different tab: note that
both tabs are highlighted and you receive the [Group] notation. When
[Group] is invoked, any change you make on one tab will be made in the
grouped tabs as well, so use caution!

De-invoke [Group] by right-clicking a grouped tabname, then select
Ungroup Sheets.