I don't think there is anything special about March 15 for individual broker accounts. A corrected broker 1099 could occur before or after that date. I have had that happen. I now wait for April, and I check the broker's website right before sending in my return. It sure is nice to be able to prepare the tax earlier and still be able to modify the return easily in TurboTax if something changes.
EPD is later this year than most MLPs. I expect this is due to their needing the K-1 information for some other partnerships, and they have to have time to use that. Last year EPD holders in effect owned pieces of 4 other MLPs, and that complicated things for us and presumably the folks preparing the EPD K-1. This year I expect that to continue, and I hope unwind the situation from the 2010 K-1.
I looked around at some of the other MLPs and found ATLS expects to have their K-1 on March 20 -- another of the later dates. On https://www.taxpackagesupport.com/atls they explain some "important events" which I presume impact their K-1 taking extra time. That is a nice touch to offer that info.
But anyway, it is common for MLPs involved in either side of a buyout or merger to have later K-1s than for those who do not. It's nice that they give the schedule to us. It is nice that we don't have to wait for the mail.