Y does a second line appear?

After you take a hpt, y or how does the 2nd line appear after the window of reading the results..... I always wonder this if the test stopped working how does it still develop after 10minutes? Almost all my test do this.... sorry if this question is silly but should I be concerned that some thing could be off balance or wrong with me?? The hubby checks my test but I cannot explain the second line is invalid .... you ladies are always helpful in all the answers.....baby dust to all 😘