I've been receiving an allowed memory size exhausted error

I've been receiving an allowed memory size exhausted error on a number of my sites in the past few days. The only plugins these sites have in common are smush pro, defender, and hummingbird. Altering the wp-config.php with define( 'WP_MEMORY_LIMIT', '200M' ); fixes the problem but I'm wondering what could be using so much memory.