It could also be related to Harry's relationship with Snape's house, Slytherin. If Harry has somehow slighted the Slytherin house or one of its important members, Snape, as the head of Slytherin, might send a Howler to put Harry in his place.
One possible reason could be that Harry has made a significant mistake in Potions class. Maybe he has endangered himself or others during an experiment, and Snape, being strict about safety and proper procedure, sends the Howler as a very public scolding.
Perhaps Harry has been snooping around in Snape's private business. Snape is a very private person, and if he suspects Harry of prying into his secrets, like his past associations or his personal research, he would be furious enough to send a Howler.
Chapter 287, "Saved", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with For five consecutive days, the search and rescue team had not... and closes on Failing to obtain any news after numerous attempts, Lu Xingchao had....
Chapter 408, "You Can't Be on the Same Team!", is the clearest early answer because it turns Flash Change's situation into a concrete plot event. The chapter gives readers a real answer by opening on Zheng Xun fell silent after he understood the situation.
Chapter 376, "I'm Just Afraid You Don't Need Me Anymore", is the clearest early answer because it turns Crown Prince Jin's situation into a concrete plot event. The chapter gives readers a real answer by opening on The phone call fell into a long silence.
Chapter 300, "You're retaking the college entrance exam?!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with I dropped out." Jiang Li said calmly.
Chapter 513, "A Public Stage", is the clearest early answer because it turns Jiang Shixu's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Everyone was sharing their experiences in a few words, the banter...
Chapter 786, "Working at Chen'an?!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with When Jiang Li left Taiqi's office building, it was already getting... and closes on She recognized that this must be the streamer Xiao Xue..
Chapter 761, "Can You Take Us In?", is the clearest early answer because it turns Aunt Feng's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Lili, this is your maternal uncle's family..." Jiang Li lifted her...
Chapter 307, "I Am Sister's Real Younger Brother!", is the clearest early answer because it turns Zhou Wenshu's situation into a concrete plot event. A useful checkpoint is the way the scene starts with From the certificates and trophies on the wall, it was clear that...
Chapter 767, "Jiang, Jiang Yu?!", is the clearest early answer because it turns Jiang Mingyu's situation into a concrete plot event. The chapter gives readers a real answer by opening on Then, Zhao Lang felt a different kind of gaze and turned his head,...
Chapter 619, "The Feast of Fish CP Is Indeed Real!", is the clearest early answer because it turns Jiang Cheng's situation into a concrete plot event.