I found a solution!
If you click on a cell and it selects multiple cells (for me it was 4-5 cells below my selected cell), then right click on the cell selection, regardless of how many there are. Go to Format Cells. The last tab in the dialog box should be "Protection" click
on it and un-select the "Locked" option box When you click OK, you can click on any of the cells you chose and it will only select that cell instead of multiple.
You may want to do this to all the cells in your worksheet so the problem doesn't continue to arise.
Yes, this works. It is a cell protection issue. When it occurs, right click in the group of cells, select 'Format Cells', select the far right tab 'Protection', then deselect 'Locked'. This unlocks cell protection.
I found that benzo95's throubleshooting was correct. If you are clicking on the last row or column of a page with the adjoining page showing, it will occur. Sometimes clicking in the footer does the same thing. Somehow these actions turn on cell protection
and the cursor begins to select multiple cells. I noticed that if I had copied a group of cells and pasted it somewhere, and then later selected a single cell within those cells, it would begin, even if I had not merged those cells together but merely copied/pasted
them as a group. Generally the size of the automatically selected group of cells was the same as the size of the cells I had copied/pasted together from somewhere else. In each case it only occured with cells that had formulas, so perhaps there is a cell protection
issue that automatically arises with formulas in multiple cells copied as a group and then pasted to another location. Maybe it's only when you do this to the last line of a page.
Yes, changing layout or zoom can solve it temporarily, but the only way I was able to eliminate it was to deselect 'locked' cell protection and then stop selecting a cell in a copy/pasted group of cells with formulas.