Looks to me like it's saying okay you have 90MB, so I tried to grab 50 or so of that but it wasn't available. I Wants Moar! Most likely something else was using about 30+ of your allocated memory at the same time. If that's the case then it's "first in, best dressed" when that happens.
If this has to do with a backup procedure, consider running it at a different time of day, perhaps when your sites aren't too busy and the memory beasts are napping. Take a look at your plugins (if you use them). Some of those things, however handy, are notoriously hungry. Caching is practically a necessity if your sites are popular too.
If you're not running anything that you think should be chewing your memory to the point of error then a quiet word to Support will help pinpoint the cause.
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