We live in a world governed by the law: cause and effect. And though there are times when we see no logical or illogical reason for a particular action (in this case anger), there is definitely reason.
We see and feel that another person was mad at us. Quickly calculate and decide: do not have any reason!
However, there is a reason. Motivated us, or something we did intentionally or unintentionally, perhaps without realizing it, or it might be externally independent of us, perhaps even independent of what is happening around us at that time.
It may be that the present action around us remind the little old trauma, an insult, abuse, ... And in his mind, perhaps subconscious level, to make a connection, a parallel. This would be a reason for anger, without us being in any way involved.
Or maybe other issues, grief, to make it the one next to us to take a certain defensive attitude, and we to interpret this as if they were mad at us.
Conclusion: Whatever the reasons for grief, they exist. Upset no reason there can only from our point of view.