Tantrums happen for many reasons - not getting what they want, stress, crankiness, situations in which they are unable to cope with (an older child snatching toys away from them), or even from hunger.
Prep yourself for such situations by reading up on ways to deal with tantrums, talking to your partner about how you both are going to handle the tantrums, and mentally preparing yourself for it. Understanding why tantrums occur and how to deal with them can help you to alleviate the situation when the time comes.