I'd be sure to charge them before storing them for the next show. Most batteries don't like being stored for long periods in a low SoC state. And I'm sure the tamper requires a functional battery.
I have to assume that one of the hard-shell cases got thrown around a little too much which caused 4 readers to go into tamper mode. 3 of the tamper mode readers were in the same case but the other was in my nicest case and still had the issue ("nicest" = custom made case vs pick-and-pluck-style).
The tampers I can somewhat understand (still just so odd to have 4 die "at once"), maybe they did get banged up, but the failed NFC and not holding a charge issues are less acceptable to me given the time/use.