contacts deleted and calendar wiped on iphone

Hey guys, 2 guys at my job have had this happen to them while they were on vacation abroad.

- Some of their contacts got deleted and some contacts they dont know got added.

- Their calendar got wiped clean

i have suspected it might have been something going on with the sync but the fact that strange contacts got added kind of ruled that out for me, anyone know why this happened and how to revert/prevent it?