It is caused by WC3 loading new data. To lower RAM requirements, WC3 does not load all the data it will use throughout game play at the map start, it loads them as they are needed. Most things in WC3 buffer the loading of data so the data is loaded quite some time before it is needed so it is in no pressure to load it. The delay arrises when something applies pressure for data, which means it needs the data yet it has not been loaded.
This results in a period of time where a model or effect or even spell is missing as it is still being fetched from the hard disc or other backing storage. Some games do not pause for this time period as you can notice by not everything appearing at once or character models suddenly apearing when they should have been visable a while before. WC3 however, as it is multiplayer, pauses (actually comes to a stop more than puases) the game while the data is fetched. After the data is fetched (for all players) it resumes the game as normal. The reason it does this is to stop in multiplayer desyncs arising due to different hardware specs loading the models at different speeds and to overall create a more reliable game engine with fewer factors that vary from PC to PC. Preloading data via triggers reduces this lag, so it might be advisable to do so in your case.
Once data is loaded in WC3, it generally can not be "unloaded" where by it is removed from RAM to reduce RAM usage. This means it should not result in any delay the next time the same opperation is preformed.