Is it possible to accidentally enable all views at once? I found all views enabled on a client site. Is it possible to enable all views at once? 

Disclaimer: I did a module and core update with drush two days ago, but didn't look at the views list.
Accidentally? Possible, but not likely. But I guess it depends what you mean by "accidentally" exactly...

There are perhaps 3 obvious ways to programmatically enable/disable a view (see https://drupal.org/node/820110), plus I'd imagine a few derivatives and/or lower level methods. But enabling a View isn't that straightforward, some manipulation of serialised PHP vars needs to happen somewhere, so I'd say the chances of it being done accidentally are slim at best.

But yes, it's possible to programatically enable all Views at once. It's also technically possible for it to be done via a DB query (i.e. outside PHP), but it would be messing with a serialised PHP array, which MySQL et al don't know how to process, so you can probably rule that out.

Now, if there's a legitimate code/process in your site that intentionally enables one or more Views in certain contexts, and it's being called accidentally, or without your knowledge, that's a different story. If that's the case you're probably in audit mode; get a backup of the site from before the update running, switch off modules until the problem disappears, and when you find the culprit, dig into the code to see what's happening.