There are 4 points to consider why doing a Lat Pulldown in front to the chest is safer and better than pulling the bar down behind the head to the traps:
1-the Latissimus Dorsi are fan-shape and angle slightly forward from the back to the front. If you lean forward in order to pull the bar behind your head, then you shift the angle of the Lats further forward while the direction of the cable (resistant) is straight down. This is not optimum power output for the imposed workload.
2-the shoulders are externally rotated too much, putting the joints at their weakest points. This is not good because you are going to put your weak, unstable shoulder joints through a lot of contractions. This will hurt your shoulders.
3-your head is tilted forward in order to pull the bar behind it. This strains the neck muscles. Why strain the Sternocleidomastoid to work the Latissimus Dorsi?
4-the bar sometimes hit the head or the back of the neck when it comes down to the back of the neck. This is absolutely DANGEROUS because this could cause serious injuries. Hitting the head and/or spine with a loaded bar, over and over again is recipe for disaster.
Thus, if you pull the bar down in front of your head to your chest, then...
1-you lean back a little which aligns the angle of the Lats with the direction of the cable which maximizes the effort.
2-the shoulders rotate to the front a little which ease up the strain on the joints.
3-your head is tucked back a little which aligns your posture upright.
4-the bar hits your chest, if it goes that far. Touching your pecs should not result in any injuries. But if you want to be really cautious, pull the bar down and stop before you touch your chest.