I have had a Nimbus Alloy Seatpost on my 36er for about 2 years or so. I recently took a pretty soft UPD(which I do not do very often) and saw that the seat was wiggling. I got off to check it and my entire seat fell off! I later discovered that this was the result of 2 cracks in the base of the seat post. I spent the rest of my ride walking 30 mins home. 
I know that this is pretty common with trials, street and flatland unis but has anybody else had this happen on their 36er or 29 road uni?
Is that another crack though the weld in the second photo too?
Crack in second picture
No, I think that it is just a scratch.
I started this short thread after replacing my Nimbus alloy seatpost with a Nimbus CrMO seatpost. There is one comment by Brian O, who is not a regular on this forum, but seems to know all about seatpost options. If you follow his “Hitting Jumps on a 36” video link, you will understand how he acquired this knowledge!
You didn’t mention muni there but I had that happen with the Nimbus muni that I bought used, about a week after I started riding it. I suspect that the type of riding matters less than how many times it hits the ground. Seatposts take a lot of shock and alumin(i)um has no fatigue limit. Maybe you could make a unicycle that would never fail no matter what but it would probably weigh 50 lbs. 
Occasionally you get a hint that something’s about to let go because the unicycle makes a different sound when you drop it, a rattle or sort of a hollow instead of solid sound. Sometimes you just get a “bummer, dude” situation. But if I had a choice between 30 minutes of pushing a broken unicycle and, let’s say, waiting in an emergency room for the same amount of time, I know which I’d choose!
A couple of weeks ago I pulled up on my seat and tore it right off the seat post. I felt pretty powerful. In 2.5 years of riding I’ve broken 4-5 seat posts. Same place as pictured, above. I’m heavy, I pull on the seat a lot, and I have handle bars on my larger unicycles. This contributes to more stress on the t-bracket of the seat post. There are some new, stronger seat posts on the market.
Over the years I’ve killed a lot of seat posts (and Nimbus Shadow Handle Bases). For mine, the main culprit was not drops, but pushing and pulling. We put quite a lot of leverage into our seat posts, depending on our riding styles, especially if they have long handles on them. Over time, this will degrade the metal and eventually lead to breakage.
For me, this has occurred mostly with thin steel posts (Miyata) and various thicknesses of aluminum. I haven’t had any thick steel posts/bases, like the Schwinn/KH-style ones break on me, though I’ve bent one.
On some of my unicycles I’ve added gussets to strengthen the joint between the post and base, which worked very well for Track racing and my older Muni. Both were more responsive to ride, and neither of them ever broke. My Track uni gusset went up to the front handle bolts, which made it very rigid.
The handle would be a major contributor to the failure for two reasons. Firstly it applies a strong bending moment to the seat post flange. Secondly it focuses the bending stress.
Notice how the crack on the front side is almost in line with the edge of the handle mounting plate.
I have always believed that mounting extended handles onto the seat with no direct attachment to the post was structurally inadequate.
Yep- I broke three of these seat posts just like this over the course of a few months after I installed KH muni handles. I was using the handle during hopping which put a lot of force on the seatpost/seat junction. The longer handle increases leverage/potential force delivered as compared to pulling up at the saddle end. I’ve switched to the KH seatpost hoping for better results- only time will tell but so far so good…
A scratch that entirely went into all the welding grooves…
…sitting directly under the seat.
I doubt it’s a scratch. But if it is, then I’m curious to learn what object must have caused it.
It is a little hard to tell from the picture, but it does appear to go into the welding grooves like you said. It’s also extremely jagged. It’s possible that it is a scratch, but it looks more likely to be a crack, based on the photo.
I have this exact seat post on my muni, and I’m going to be keeping a close eye on it for sure!