Ham string cramps

I did IMFL last november. When I stood up getting out of the water on the first lap, my hamstrings cramped up. It took a couple of minutes to get rid of the cramps, then got back in the water, and did not have an issue after the second lap. So I did not think anymore about it. Just one of those things that happen, and move on.

Then I did an Olympic distance race this weekend with a 1.5K swim. Same thing happened. Cramped up on the hamstrings getting out of the water. I walked it off slowly, and had no problems on the bike or the run.

I don't have this problem in the pool.

Any ideas on what is causing this and or what to do to prevent this?