It won't actually matter whether you understand the reason, because it's not something you can remedy and rule out for sure. It could be caused by a cache problem (not something you can address).
The thrust is, it's a prob that has happened and could happen again, and the question is whether you can deal with the consequences if it does. If not, your options are to stop using email on iPad or to stop sharing an iPad.
Ipads (and many other brands of mobile devices) don't allow individual logon profiles (unlike laptops and PCs), that's the key thing.