1) There was fog, probably thick but patchy, at the time of the accident. This stretch of line alongside the Meuse is notorious for fog and there had been significant thunderstorms earlier in the evening which would have produced perfect conditions for the formation of fog. The driver of the passenger train almost certainly didn't see the red signal and running into the back of a fully-laden stone train would have been akin to hitting a brick wall.
2) Although Infrabel (Belgian NR) are remaining tightlipped on the subject it would seem that at least some of the signals in this area are not yet fitted with TBL1+ despite claims that 98% of the (B) network (and around 75% of actual signals) have been so equipped already - despite the fact that the propensity to fog*, the mix of traffic (passenger + heavy stone trains) and the previous collision at the same spot less than 8 years ago should all have combined to make it high priority. If, in fact, the signals have been equipped with TBL1+ (press speculation in face of Infrabel's silence), then it clearly wasn't operational at the time of the accident, possibly knocked out by the earlier storms and not reset when the signalling was.
3) Unlike the UK, it is normal for accident investigations in Belgium to be made public as they proceed so Infrabel's silence isn't being well-received. If it does emerge that the fitment of TBL1+ to this line wasn't accorded the priority it should have had, there will be strong suspicions that it was pushed down the list because it was in French-speaking Wallonia and not in Dutch-speaking Flanders. It is notable that both the King and the Prime Minister felt the need to attend the first press conference!
* Fog in the Meuse gorges can cause such serious problems that further upstream at Namur the Beez Viaduct carrying the E411 autoroute is fitted with permanent "fog signalling" and special lighting.