As others have said - you’re definitely exceeding the allowable working load of the belt. You can find in this document from SDP-SI on pg T-15 that the recommended working tension on 1" width of 5mm pitch HTD belt is 102lbf or 454N. Again note this number is for 1" width and you’re only using 15mm (0.59"). You can do some math to determine what your belt working tension is using the pitch diameter of the belt, wheel diameter and traction limited point of your wheel (based on CoF of wheels and weight on the wheel). Of course this load is meant if you want your belts to last a lot longer than they need to in FRC, and there’s some safety factor built in.
Some rough math:
Robot Weight: 150 lb
Wheel CoF: 1
Wheel Diameter: 4"
Number of wheels bearing weight: 4
Max torque / wheel before breaking traction = 150 lb / 4 wheels * 2 in = 75 in lb
Pitch diameter of pulley = Tooth Count * (5 mm pitch / (25.4 mm/in)) / pi
An 18T pulley has a PD of 1.127" or a radius of 0.564"
Max working load on belt = Max Torque / pitch radius
For an 18T pulley this means you’re loading the belt with (75 in lb / 0.534 in) which is 133 lbf of tension.
Additionally you’ll notice the belt that failed transmits all the torque to two wheels. The more weight you have on wheels drive off a single belt the more load you can apply to the belt before your wheels slip. If your weight is shifted onto the two wheels driven off that middle belt you may be putting half the weight of the robot on the wheels driven from a single belt allowing for 266 lb of tension!
You can help reduce this load if you run a belt from the gearbox output to the far wheel instead of using the middle belt to transfer the torque to the outer wheel. But really you want to run at least 24T pulleys if not larger. We typically run 24T pulleys on the gearbox output and 42T pulleys at the wheel.