Your plugin has been great to use up to this point, so thanks first for you hard work: much appreciated.
I've read several of the recent support tickets and I believe the memory issues that we are all experiencing are very likely related to Updraft Plus. Update: I'm on Version 1.7.18 of UpdraftPlus
I can't confirm exactly when I upgraded the UpdraftPlus plugin on my site, but it occurred sometime earlier this week, around the same time that I upgraded to WP v3.6.1.
When I look at my site resource usage since then, I appear to see a massive spike in memory use around the time that Updraft plus kicks in for its nightly backup, however this memory allocation is never released. The following day the pattern repeats again, and the memory use doubles again.
In my case, I am currently running at a constant 962,936KB of an available 1,048,576 KB. And additional use of the site periodically tips that over the allocation limit, killing site access with out-of-memory errors.
Note that prior to the WP v3.6.1/ UpdraftPlus updates, my constant memory use ran extremely low: average was close to zero, and peaks comfortably under 256MB. The first backup took this to a constant average of over 512MB; the second to over 768MB, meaning that any reasonable use of the site now causes memory errors.
I've finally managed to get back into WP Admin between out-of-memory occurrences, and have disabled UpdraftPlus, however the memory hasn't been released as I believe it's locked up in the cron jobs that apfotos reported here:
David, I believe that - all things being equal - it is very likely UpdraftPlus causing the problem, possibly in combination with WordPress v3.6.1. I strongly recommend/ advise investigating this.