If it is city water it might be from high flow caused by a line break, new construciton, or even a fire.

Check with the city.

1- Exactly like Bill said

OR

2- Your own piping is ferrous metal and starting to give up the ghost

3- A cross connection where someone upstream contaminated your domestic water supply

4- Bad well water

5- Someone closed then opened your water main

6- Broken water supply allowing soil to enter your water via siphoning action.

7- Mixed ferrous and non ferrous metals