Those conditions allowed more infections to happen, and eventually one of them was more deadly. ANY situation that results in more infections will result in more mutations (as it is a property of the viral replication mechanism), and increase the odds that one of them will be more successful in infecting and spreading - with the possible side effect of host death.

What some folks are proposing in this thread (uncontrolled community spread so it will 'mellow out') is an even more massive expansion of that due to the quite visible ability for COVID to be infectious in patients with no symptoms for long periods of time, which Influenza has not been able to pull off.