mjcmjc4321, Your examples appear to have slightly different meanings. This is how I understand the difference. 1) Then the store manager, depending on her mood, might unload on (give a tongue-lashing to) the manufacturer... Here “unload” carries the idea of a sudden outburst. It is related to the definition of “unload” meaning “to fire off a gun”. “He unloaded (shot) several rounds from his shotgun into the charging bear.” -The coach really unloaded on [=yelled at] the players. -Briana unloaded on me and told me what she thinks of me. -He unloaded a vicious attack of punches on his partner. (The boxer began a short powerful attack.) 2) Don't worry about it. We unloaded a lot on you. “Unload” here is related to the idea of unloading a ship or truck. One person unloads himself of thoughts or emotions by communicating them to another person. The load can simply be a lot of information, or it can be heavy emotional information. -On the outside she seems to be very content with her life. But one afternoon she “unloaded” everything she had been storing-up. -Instead of the three-minute comment he was supposed to have prepared, he unloaded a 25-minute Soviet-style speech. -He found relief at last in telling the story to his good friend, John, upon whom he unloaded it with sad results.

In this context, "unload" means to tell someone your troubles or concerns. The idea implies transferring or "unloading" an emotional burden to another person.