All datapacks are reloaded when using /datapack enable xxx or /datapack disable xxx.

For example, as the attached file (two datapacks, named 'a' and 'b'), they both have a function named init (in different namespace), which just say a in the first init.mcfunction and say b in another. And they both have a 'minecraft:load' tag.

At first, use /reload. It gives

[Server] b
[Server] a

which is normal.

However, now use /datapack disable "file/a". It gives

[Server] b

which is weird, since I did not do anything about b.

Now, use /datapack enable "file/a". It gives

[Server] b
[Server] a

which is also unreasonable, since I still did not do anything about b.

My guess is that Minecraft reloads all datapacks when using /datapack enable xxx and /datapack disable xxx command.