The burn was scheduled to last 50 seconds. In that time, the probe was to accelerate from orbital velocity of roughly 17,500 MPH to a escape velocity of over 25,000 MPH. That's a pretty healthy acceleration but actually less than the peak loads experienced during launch.

All that's known at the moment is that the burn started and the probe was never heard from again. Two pieces of apparent debris were tracked along the projected flight path, suggesting some form of breakup. It'd be interesting to see if they can determine how long the burn would've had to be in order to achieve the trajectory that matches the debris. That would give some insight into the nature of the failure.

Solid rocket motors are very reliable but they do fail from time to time. There are several known failure modes for the engines and each will have to be investigated.

Most often, we hear about the big failures like Challenger or the one on the Titan 34D Stage 0 engine back in April 1986 or the Delta II strap-on a couple years ago. A lot of the smaller failures may not get widely reported.

For solid rocket motors, the most likely failure modes are due to cracks or voids in the propellant, defects in the engine casing, or insulation problems. A crack or void in the propellant exposes an excessive amount of surface area to the burn, causing excessive pressure. In most instances that I've read about, this usually happens pretty soon after ignition, which doesn't appear to match the trajectory of the debris, unless it was a crack near the end of the propellant.

A defect in the engine casing probably wouldn't take too long from ignition to failure, so that seems not to match the trajectory, either.

I still think the most likely scenario based on what I've read so far points to insulation problems. The insulation keeps the heat from the engine casing, protecting the rocket in two ways. First, it protects the casing from burn-through. Second, it keeps the heat from propagating through the engine casing and prematurely igniting the rest of the propellant, causing an overpressure leading to case rupture. Depending on where the insulation problem occurs, it could happen shortly after ignition or well into the burn.

It'll be interesting to see what preflight inspection was conducted on the rocket motor. Many techniques are used to inspect for voids, cracks, and insulation debonds, including X-rays and ultrasound. I wonder if the length of time between launch and ignition could've been a factor. Most solid rocket motors used in spacecraft are lit within a few hours to a couple days of liftoff. This engine had been circling the Earth for a few weeks, undergoing many thermal cycles during the day-night portions of the orbit roughly 16 times a day. It's possible that might've caused a debond or propellant crack.