Simply stated, tubing is porous and readily absorbs moisture from the air.  Depending on the type of tubing on your synthesizer, and the length of each line, a regular bottle of solvent can absorb a lot of water in 24 hours.

This is why the use of Molecular Sieve Solvent Pouches is so important.  Open and close a bottle a few times, leave reagents on the machine over the weekend, incomplete priming of the dispense lines, etc. and you can have pretty wet solvents.  The MSSP will help repvent this "issue" from becoming a disaster.

As a reminder, water is better nucleophile than any activated monomer. Water will out compete the condensation reaction of "amidite coupling" and terminate your synthesis progress at any reactive sites on your support or growing oligonucleotide chain.  

Below is a chartof standard absorption rates of common tubing found on DNA/RNA synthesizers: