Reversed polarity of CR2025
If the two cells are in series but one has significantly less capacity that the other, it can discharge first. The other cell will then discharge the expired cell even below zero as you have seen. If cells are in series you should ensure they are in the same condition, do not pair a new cell with a partly used one.

This same effect can occur with rechargeable cells and in that case the cell that reverses polarity can be damaged permanently. You should always be careful about over discharging a series battery.