This is totally normal. There can be a couple of reasons for it happening.
First, they are exploring their environment. There is something new in their mouth and they want to feel it with their tongue and hands.
Sucking on their hands is also a natural reflex. My babies suck ed (one still does because he's just a little guy) their hands as a comfort thing and to explore. That's why babies put EVERYTHING in their mouths. They like to explore and learn with all senses.
The other reason is to help them move food from their mouths down to their throats. Babies at that age are not able to transfer food to swallow it easily. Babies' digestive systems are not really prepared to break down food easily until they are about 6ish months of age. That's why they make such a mess during feedings as well.
Don't worry. Other than it being messy (and gross, lol) it's completely normal.
God bless,
A.