Sometimes it's hard to tell anger and sadness apart. Those two emotions are very different but they can disguise as each other very easily. Consciously or subconsciously, many people hide their sadness by expressing false anger or hide their anger by expressing false sadness. Sometimes those people can even convince themselves of the false feeling.
What causes someone to hide their feelings depends on the person. One of the most common reasons is fear. Fear of what reaction expressing their true emotions may cause on other people. Sadness often causes people to feel and be weakened, and having their weakness noticed by others can be very scary, specially by people they don't trust enough. Anger, on the other way, does the opposite. Anger makes them feel stronger, and it gives them an intimidating impression on others.
In conclusion, some people disguise their weakness (sadness) as strength (anger) in order to avoid being manipulated or abused. Similarly, some people disguise their strength as weakness in order to receive less fear and more empathy from others. How well this works depends on how well they can convince others and themselves of their false emotions.
One of the main consequences is that people who do this, on purpose or not, may lose control of their feelings. It also may give them a false sense of comfort. For sadness disguised as anger, thinking they're strong won't make them any less weak. For anger disguised as sadness, thinking they deserve empathy won't make people more empathetic to them.