See summary. Probably a result of copy-pasting the code from the zombie into the giant.

How to reproduce:

1. Place a bucket of water on the ground
2. In the center of the water pool, summon a giant with DrownedConversionTime set to 100
3. Watch and wait.

According to the Minecraft Wiki, issues about the giant mob are not supported, but I've been told to take the wiki's opinion with a grain of salt.