Why are some of my tiles blacked out?

[WPCentral Question]

About 20 per cent of my tiles that I had "pinned to start" have gone completely black.

 

[gpobernardo]

I've encountered this quite a number of times, but it happens only after I exit a "heavy chat app" that seems to unload a lot of data from RAM. If possible (without violating your privacy), can you post a screenshot of your start screen, or tell us more of what happened before your tiles blacked out?

 

[Mumblic]

About 20 per cent of my tiles that I had "pinned to start" have gone completely black.

Are these application tiles or shortcuts to webpages pinned from within Internet Explorer. Because I have a similar problem with theses shortcuts. They start as a normal tile with a preview of the page. They turn blank when more webpages are pinned to the start menu!

 

[Squeegies]

or maybe it's just that you removed your sd card where you store your apps?

 

[Zulfigar]

Could also be your phone is doing a backup and the backed out apps aren't usable during it because they're being backed up as well.

 

[dot-the-eye]

Thanks, all, for your replies.
What turned black are shortcut tiles to webpages.
What I did to restore them to original look was select each black, saved the webpage as a favorite, unpinned the black, selected the favorite and pinned it back onto start.
Through the overnight, the renewed ones held on.
I do notice that the MSN News shortcut, which before presented changing updated photos, now is the standard, unchanging block white-on-red look.
I hope they don't start turning black again. What are the chances?
Lumia 920.