Short answers: Yes it's coincidence and random bad luck and no, there is no apparent enemy limit.
Detailed answers:
Having played through this game many times (mostly on Hard) I don't think I've ever encountered burrowers on the 1st island. In terms of enemy patterns/order of Vek appearance, I think it really is random with the only exceptions being that a few of the more powerful enemies (for example: burrowers, diggers, blobbers, crabs, and maybe centipedes) will not to my knowledge appear on the 1st island but some of them may start appearing on the 2nd onward. Alpha enemy versions can appear at any time (tougher difficulty makes them arrive sooner, even in your very first turn of your first battle on hard). Otherwise, it's really a roll of the dice. I, too have experienced incredible bad luck with Steel Judoka runs where burrowers (among my most hated enemies overall and the absolute worst opponents for this squad as you well know) were on two islands. They've' cut several of my perfect run tries short. The game selects the enemy grouping for each island at random, and it will change for some islands as you complete others to start adding in the tougher Vek opponents later on (you can see which ones are there are before choosing an island by mousing over the island). I think each island's enemy content only changes once, but I'm not sure about that. Try to save burrowers for last with this squad as you may be able to pick up a weapon from another island that helps you handle them better.
In terms of enemy number limits, no, I don't believe there is any hard limit on the number of enemies you will face in any given battle or overall with one exception. There is a mission on Pinnacle that starts with all enemies that there will be already on the screen but frozen; no new enemies will spawn here. On all other missions, you can try to limit the number of enemies by blocking spawn points, but the Vek will keep coming until you reach the final turn of the battle. Fewer Vek will appear on lower difficulty, but I still don't think there are any limits (especially considering the final battle will continue with a constant stream of new enemies until either your squad is defeated or the bomb goes off). In general, blocking Vek spawns and killing Vek will result in more Vek spawning to keep enemies on the battlefield. I think that the total number of enemies (excluding enemies spawned by other Vek/buildings such as spiderlings/bots) is limited to about 7 at a time on the battlefield.
Good luck!
