As soon as the market heated up, every company jumped in at once.
Tips
usage
Almost always critical — implies herd behavior without planning or organization. Typical pairing: 盲目 + 一哄而上. A common warning in business and policy writing.
mistakes
Do not confuse with 一哄而散 ('to scatter all at once'). 上 = surge forward together; 散 = scatter at once. Same 一哄, opposite direction.