High volume does not necessarily mean high buying demand - it could mean that there is high supply, meaning that there are a lot of sellers wanting to dump the stock.
There could be lots of reasons for higher volume - perhaps the stock was just added to some index? Or that there was recent news that increased activity but an equal number of buyers and sellers? Or that there are panic sellers for the short term but lots of buyers for the long term.
There's not really a fundamental reason that can be applied in all cases.
In this particular case (GitLab), my interpretation is that earnings were announced, the market wasn't too happy about it (it dropped 30% overnight) but there seems to be a consensus on what the fair price is. Many investors are probably cutting their losses at the going price but not necessarily panicking, and there are many that seem to like that price and are willing to buy.