It's true that passenger traffic has increased and in some place that can create problems.
But there is a general reduction of useful infrastructure because Infrabel has decreased the number of railroad switches. Historically, I think the network has a lot (too much ?) of switches but the reduction was maybe too large.
Infrabel has also reduced the number of voies accesoires and of voies de faisceau (sorry, too tired to find the translation) => decrease of possibility to park a freight train => less degree of freedom to draw the pathways for freight => less capacity
Moreover, even if it's "temporary", there are a lot of works on the network.