You mean they're bubbling up? We had that problem before in the summer time at our old location, where they were on concrete. I think the concrete transfers the summer heat to the mats, and the warmth makes them expand. If they're right up against the wall on multiple sides, there's no room for them to expand outward, so they bubble up instead. We solved it by cutting maybe 1" off the far edge of the matted area. It took a few weeks, but the bubble slowly went down.
The closest I have experience of is with laminate flooring (interlocking engineered planks of MDF with a wood facing).
On installation you have to leave a gap on every edge (say 1/2" in an average 18'x12' room) and cover with a finishing strip to allow the flooring to expand and contract with the variation in temperature - moisture plays a part too, it's recommended to unpack the boards in the room where they'll live and leave them for a day or so. They're manufactured very dry and the normal moisture in a house causes expansion.
Without the gap you get a little hill in the middle of the room (or splitting if you fix the edges) and the joins are where it's worse because those are the interruptions in structure.
Moisture again, warm summer air can carry more than cold winter air and concrete underneath can cause more to condense out into the covering - different underlay materials (or none, which is worst) make this more or less apparent in terms of expansion.
Whether or not the mats in question are quite as susceptible to moisture and temperature as engineered wood I don't know (the 1/2" kiddy type jigsaw mats I have certainly move a fair bit), but I think it's worth considering.