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.
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.
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.
Chapter 612, "There's been an accident on the show!", is the clearest early answer because it turns Gu Shengrong's situation into a concrete plot event. Early on, His family's residence used the mortise and tenon structure in many....
Chapter 703, "Slap Oneself", 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 Wu Daoshan quietly raised his head to size up Zhao Lang, and sure...
Chapter 519, "I'm Here to Participate in the Program", is the clearest early answer because it turns Younger President Jin's situation into a concrete plot event.
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 431, "Su Yinwan Is Here Too?!", is the clearest early answer because it turns President Shang's situation into a concrete plot event. A useful checkpoint is the way the scene starts with [Holy shit, why is Su Yinwan on this show too???] [!!!
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 265, "This Time You Must Help Me", is the clearest early answer because it turns Weibo's situation into a concrete plot event.
Chapter 581, "Where is everyone? Where have they all gone?!", is the clearest early answer because it turns Are's situation into a concrete plot event.
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.