It's simply because you're having to spend time together 24/7, quite literally. Previously even if you lived together you had other outlets and could easily ignore things that annoyed you.
Previously you'd have work, out of house hobbies, social events etc giving a bit of breathing room. A manageable minor difference can become much more pronounced in lockdown.
The longer a couple has been together the more likely this is not fixable, and once things turn sour you can't really turn back. Thus we're seeing the end of longer term relationships with relationship counsellors staying it's to be expected and there's not much you can do about it.
By comparison those same counsellors are trying to persuade newer relationships to ride it out because of the exceptional circumstances. If it's a younger relationship these disagreements are more likely to do with miscommunication or resolvable issues that are accentuated by the excessive time spent together. They generally would not be guaranteed to appear as the relationship goes on and are unlikely to permanently damage the relationship.
However the problem they've noticed is people in newer relationships are unlikely to use Counsellors. Secondly, lockdown potentially can completely annihilate the "honeymoon" period, meaning people are extremely demotivated and tend to completely give up on the relationship.