I just noticed this yesterday while comping some horns. Some takes the horn player would play longer than others.
Example:
Three minute song
take one from minute one to song end
take two from minute one to minute two
 
then looking at the take lanes - the top one (which is the last one recorded)ends at minute two - the first one right below it ends at the song end. When you start comping the last recorded take defaults to be the selected material until you edit it differently. So Sonar splits the first take exactly where the second take ends so that it can promote the lonely last minute of the first take to the defaulted selected material alongside the second take.
I noticed this because I went to link the common takes from three different mics so I could comp one and the others would follow suit; but I was confused that some takes had splits (I thought they were overlapping multiple takes at first) I was just linking three takes but ended up linking five or six groups of split separated clips.
After I figure this out I selected all the clips in each take and bounced them to clips which left me with one clip for each take no matter the length of the take. This left me with what I would expect when comping . 
Does this automatic splitting of different length takes while comping help anybody? Can I turn it off?

You can't turn it off that I know of. It is just the way takes work. You can "heal" the splits just by doing the comp drag all the way across the longest take and it will promote it to be the audible take and make all the takes be continuous clips.
 

I dont belueve this is supposed to be normal...

It is supposed to happen in some element of comping mode but should not be happening itherwise.

Ive experienced it as well... PITA... for the moment.

Have you sbmitted a proper problem report?