Depending on what your actions are, the ribbon tabs can and do change.
If you insert a new table, two context sensitive tabs appear on the end of the ribbon and the Table Tools/Design tab is activated.
If you insert a picture, Picture Tools/Format appears and is activated.
I presume there are other innocuous events that cause ribbon tabs to change focus.

You can also change ribbon tabs by inadvertently pressing the Alt key and then a letter. For instance Alt followed by N will activate the Insert Ribbon tab.

If you are seeing the tab change as you mouse over the tabs then perhaps your hardware can be a factor. If you are using a touchpad or a mouse with a very light button it can be easy to click without noticing.

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Andrew Lockton
Chrysalis Design, Melbourne Australia