I don't think I'd call it a mass exodus, but we recently had three TM's on the flow team plan to all leave at once. They were going to transfer to another Target (where they heard it was a lot "better"), move to that city and get an apartment together. One was experienced and was an unofficial leader on the flow team, but had trouble following direction from his TL and ETL as he thought he was an expert of the process (he was also educated and intelligent, but full of himself). Another was an experienced flow/backroom TM who worked a lot of hours. The other was an average flow TM.
Only two actually left. The average TM stayed, and is trying to develop to promote. The educated flow TM left Target and is working as an unpaid intern somewhere. The flow/backroom TM is working backroom night shift exclusively at his new store. He says it's fun. After losing two of the three TM's, our flow team is doing better than ever and is back to coming clean every truck day, something they hadn't done in months.
TM's do a lot of talking about leaving en masse, with the belief that their store will... I don't know. Capitulate and offer raises all around? Beg those TM's to stay? In the end, not everyone leaves, and those that do, get replaced. None of us are irreplaceable, whether you work front end, flow, AP, or are an ETL. There's always someone else coming along looking for a job.