the lipos can 'pillow' from over disharge and over charging and they should only every be charged at 1C, so in your case a max charge of 4.9a. so technically charging at 5amps is overcharging. i'm not sure how much over the 1c you have to charge before they will go like that.
either way for it to happen suddenly like that sounds like it is over dishcharging. You should never let the cells drop below 3v per cell so if you can check the voltage give it a try (with a voltmeter not by putting them on charge lol) and see what the voltage is, if its below 6v then you know thats where the problem is and as jimmy says make sure that you have lipo cutoff enabled on your esc and if your esc doesn't have that function invest in an external lipo cutoff, a much cheaper solution than ruining cells every week.
as for your old cells the safest thing to do is to put them in salt water which will completely discharge them and make them safe, i'm not sure on the exact way of doing this (how much salt to use or how long to leave them in there but i'm sure there are numerous guides on tinterweb).
HTH
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