question

"Lagging behind" in level 0 in the context of LeveledCompactionStrategy means that a node has not been able to keep up with the required tasks to compact the SSTables and it has fallen behind.

When there are more than 32 SSTables in level 0 because LCS has not been able to keep up with the required compaction tasks, the LeveledManifest.java class will temporarily switch to STCS:

    private static final int MAX_COMPACTING_L0 = 32;

    private CompactionCandidate getSTCSInL0CompactionCandidate()
    {
        if (!DatabaseDescriptor.getDisableSTCSInL0() && getLevel(0).size() > MAX_COMPACTING_L0)
        {
            List<SSTableReader> mostInteresting = getSSTablesForSTCS(getLevel(0));
            if (!mostInteresting.isEmpty())
            {
                logger.debug("L0 is too far behind, performing size-tiering there first");
                return new CompactionCandidate(mostInteresting, 0, Long.MAX_VALUE);
            }
        }

        return null;
    }

This is done to quickly reduce the number of SSTables in L0. If you recall, the goal of LCS is to coalesce the partition fragments so that the least amount of SSTables need to be read to satisfy a request. This is problematic when there are too many SSTables in level 0. Cheers!