why does repeated freezing and thawing increase bacterial growth in meat?

Answer 1

One of two reasons.
Repeated freezing and thawing may cause cell damage to the meat, releasing nutrients, making them more available to bacteria.
Repeated thawing increases the time that the meat is at a temperature that bacteria can grow. It's the same as leaving the meat out for some time, except it is done in small stages. The bacteria are not destroyed by freezing, nor is any toxin they may produce.
Many food poisonings also occur because meat is stored incorrectly in the freezer. If you are putting unfrozen meat into a freezer, make sure it is not touching any frozen meat, as it will cause it to thaw out and refreeze. If this is done a few times, what you could have is spoiled meat.