Why does Internet Explorer Security Level change automatically?

In the past several weeks, I have noticed that the Security Level on Internet Explorer changes from Medium to Medium-High automatically for no apparent reason. I noticed this when several websites (including Yahoo Finance) stopped opening automatically but instead asked for the webpage password each time I went to the website. I eventually discovered that I could fix that problem by changing the Security Level on Internet Explorer from Medium-High to Medium. (The Security Level for Internet Explorer can be accessed at: Tools-Internet Options-Security)
Then this morning it happened again: Yahoo Finance asked for my password. So I went to IE, and sure enough, the Security Level was once again set to Medium-High. So I changed the IE Security Level back to Medium and Yahoo Finance would once again logon automatically. I also noticed that my Windows 7 system is running Internet Explorer 10. I thought I was running IE 9 so I don't know when the IE version changed.
So the question is: why or what program keeps changing the Security Level on IE from Medium to Medium-High, thereby causing a number of websites to ask for passwords, and how can I prevent these changes to the Security Level?

Thanks for any suggestions.