Why did this happen

[shoottx]

The weekend projects were all shop related. In an effort to have several hold down blocks available for my DP, I was routing slots in 3/4 in oak 3/8 wide and 3/8 deep. I had cut several and all of a sudden BANG The router stopped cutting and I found the broken pieces in the router cabinet. I was not aware of any undue stress on the bit or pinching of the board. Here are the pieces

[LCHIEN]

the cross section of the bits is actually very small due to the flutes and the carbide (these look like solid carbide bits) is relatively brittle.
So the recommended cut depth is something like 1/8" for 1/4 and 3/8" spiral up or downcut bits. You have to make more passes. And don't push so hard.
If you try to make 3/8" deep but push way slow you will end up burning your work. If you try and cut 3/8" deep then and push hard you will break your bits. DAMHIKT.

[Stytooner]

I consider all smaller router bits and end mills as consumable. Same with drill bits. When they go up in size past about 3/8", they are an investment instead for anything other than a pro shop. This means they will generally last longer and not be prone to breakage like the smaller bits are. Different styles and materials bits are made of will add or remove tool life as will materials and depth of cut used. Feed speed and rpm's can also play a roll.
Even when doing everything right, small bits can just snap for the reasons Loring mentioned. This is why it's a good idea to wear safety gear.

[MikeMcCoy]

A couple of years ago I had one snap (and I was doing everything correctly) while I was making the last pass on a slot and had one snap. I gave me a pretty good glancing blow on my upper cheek just under my eye. Ever since then, I wear safety glasses and face shield for anything that's not covered by stock.

[Uncle Cracker]

Ditto Mike's suggestion for the face shield, in addition to safety glasses. Carbide bits are very brittle, and do sometimes break, even when used properly. Anything that breaks whilst spinning at 30,000rpm is inherently dangerous... I've even had to use a needlenose to pull a 1/4" shard of router bit out of my otherwise ample belly... Let's be careful out there...

[cabinetman]

It could be possible that the bit was steel, and not even solid carbide. From the looks of it there was sustained heat. As said, taking less of a bite, and not pushing will let the bit cut better. I get better performance from carbide tipped bits.
.

[herb fellows]

Was it a cheapie bit, a mid range or a 'better' one? While all have their uses, the cheapie bits are a lot more prone to this than even the mid range ones. Bottom line is that it takes better steel and better, more expensive processes to make a better bit.

[shoottx]

I
Even when doing everything right, small bits can just snap for the reasons Loring mentioned. This is why it's a good idea to wear safety gear.

Always wear safety glasses, my regular glasses are for shotgun shooting and as such are better than most off the shelf safety glasses.

I think it was a cheaper bit.

I usually take a couple of cutting passes if I think the cut is too deep. May have just recalibated what is too deep